1; Antelope Canyons, Arizona; 72 photos + samples of Gallery Wall layouts; This collection of abstract square Images is perfect for creating a Gallery Wall; Slot canyons, such as Antelope Canyons, are known for their unique rock formations, textures, and changing colors. The orange and pink colors of Navajo sandstone change due to the way the light interacts with the forms. In the upper part of the canyon, only pink and orange hues are reflected. As light reflects off multiple layers dipper into the canyon, it loses the red hues with each bounce. Since blue is the shortest wavelength of visible light, it becomes the predominant color that is scattered in poorly lit areas. This effect is known as diffuse reflection and contributes to a decrease in the intensity of the reflected light. When the little reflected light is left, the light from the blue sky takes over the reflected light and creates the effect known as Navajo Rainbow. As the sun moves throughout the day, the light and shadows in the canyon change, creating a constantly shifting and mesmerizing display. ; Captured beauty of these natural phenomena, will make excellent additions to home and office decor.; Creating a composition of several square photos in different sizes can be a great way to add visual interest. This technique, known as a gallery wall, can showcase a collection of photographs and add a personal warm touch to your living or working space. You can select the images you like the best and we will help you with determining the right layout and sizing for your home or office. you can find several sample layouts at the end of the gallery.
72 photos + samples of Gallery Wall layouts1; Japan Wildlife in Winter; 157 photos;
157 photos1; Minimalist Winter Landscapes. Japan; 64 photos;
64 photos1; Patagonia's Mountain Landscapes; 87 photos; ; ; ;
87 photos1; Chicago, Illinois, USA 2021; 114 photos; All photos in this gallery were taken in 2021 and reflecting newly developed buildings such as St. Regis Chicago (previously Vista tower), One Chicago, 1000M, and others; Chicago has its own soul and individuality. In this city, one can admire three different periods of the world's architecture simultaneously. We can ‘thank' for that the famous 1871 fire, which almost wiped Chicago off the face of the Earth. After the fire Chicago was built practically from scratch, making it possible to turn into reality the boldest architectural experiments of that time. First skyscrapers in the USA appeared in that very city: the land price was very high, so the city had no other choice but to grow up. Nowadays, Chicago takes second place by the number of skyscrapers in the USA. But Chicago is not just a city of skyscrapers. The main part of it consists of 2-3 story buildings including those of a completely classic style. There are beaches and parks, including famous Grant and Millennium Parks. The city sculptures are also representing in all genres of art from classics to high-tech. There are no words to describe the charm and splendor of Chicago — it is worth seeing. Turn on the sound on your desktop and hit the "slideshow" button. Hope you'll enjoy it!
114 photos1; Chicago. Illinois, USA 2010-2020; 123 photos; City of Chicago. Illinois, USA; Chicago has its own soul and individuality. In this city, one can admire three different periods of the world's architecture simultaneously. We can ‘thank' for that the famous 1871 fire, which almost wiped Chicago off the face of the Earth. After the fire Chicago was built practically from scratch, making it possible to turn into reality the boldest architectural experiments of that time. First skyscrapers in the USA appeared in that very city: the land price was very high, so the city had no other choice but to grow up. Nowadays, Chicago takes second place by the number of skyscrapers in the USA. But Chicago is not just a city of skyscrapers. The main part of it consists of 2-3 story buildings including those of a completely classic style. There are beaches and parks, including famous Grant and Millennium Parks. The city sculptures are also representing in all genres of art from classics to high-tech. There are no words to describe the charm and splendor of Chicago — it is worth seeing. Turn on the sound on your desktop and hit the "slideshow" button. Hope you'll enjoy it!
123 photos1; DeadVlei & SosusVlei, Namibia; 115 photos; Warm Sands of Namibia; Situated in the largest conservation area in Africa (the Namib-Naukluft National Park), SossusVlei is possibly Namibia’s most spectacular and best-known attraction, characterized by the large orange dunes that surround it. The dunes in this area are some of the highest in the world, reaching almost 400 meters, and provide photographic enthusiasts with wonderful images in the beautiful morning and evening light. Close to SossusVlei, DeadVlei is a clay pan characterized by dark, dead camel thorn trees. The pan was formed when the Tsauchab River flooded and the abundance of water allowed camel thorn trees to grow. Then the climate changed and the sand dunes encroached on the pan, blocking the river. The trees are estimated to be approximately 900 years old, but they haven't decomposed due to the dry climate. DeadVlei is a paradise for photographers as the contrast between the pitch-black trees, bleached-white pans, rusty-orange dunes, and deep blue sky make for incredible images.
115 photos1; Fall Colors, Upper Peninsula, Michigan; 103photos; The Land of Forests and Waterfalls; The U.P. Michigan is home to more than 300 waterfalls, beautiful rivers, and dense forests of maple, oak, aspen, birch, ironwood, beech, ash, and basswood. This pristine land is one of the best destinations in the US for fall colors. No matter where you go in the UP the peak color season is magnificent.
103photos1; Greenland. Ammassalik Fjord, Kulusuk; 38 photos; Midnight Sun's Magic; Kulusuk is a small outlying island in the Ammassalik archipelago of islands on the coast of the North Atlantic. The island is home to Kulusuk, the fourth-largest settlement on the entire eastern coast of Greenland. Northern lights, are commonplace in Greenland, but when you think of this arctic territory, you most likely never imagine a beautiful sunset there. With iceberg debris floating along the surface of the Greenland Sea at the cross-section of Canada’s Labrador Sea and the Norweigan Sea, rays of sunlight can reflect an amazing portrayal of fire on ice in the twilight of the night. From Kulusuk towards Angmagssalik Island and across the Torsuut Tunoq sound, dramatic sunset views are nothing short of magnificent. The distilled natural elements of Greenland’s arctic scenery fuse these breathtaking scenes together for spectacular panoramic captures. The rocky landscape formations with icy conditions and crystal waters reflect Mother Nature’s true natural beauty. Kulusuk is home to many great hunters that provide the village and their families with birds, seals, and all kinds of Greenlandic delicacies. This is a huge part of the Greenlandic culture.
38 photos1; Greenland. Ilulissat Icefjord, Disco Bay; 30 photos; The Iceberg Capital of the World; The Ilulissat Icefjord (Kangia) is one of the most well-known and popular tourist attractions in Greenland. Fed by the Sermeq Kujalleq Glacier, this 61km long fjord is filled with icebergs of all shapes and sizes and was inscribed on the UNESCO World Heritage list in 2004. Sermeq Kujalleq is the fastest flowing glacier in Greenland, which is only surpassed by the great Antarctic and spits out gigantic icebergs that follow the 60 km fjord out into the bay. This fjord is a photographer’s delight, characterized by endless days illuminated by the Midnight Sun. It is a magical place to experience from both land and water, with photographic opportunities in abundance.
30 photos1; Greenland. Scoresby Sund fjord system; 97 photos; Deep fjords and vast glaciers, ridged mountain peaks and huge icebergs, Musk Ox and Polar Bear, unspoiled nature and a deafening silence ...; The Scoresby Sund (Kangertittivaq) in East Greenland is the largest fjord system in the world and the home to one of the most dramatic landscape settings on earth. Icebergs like in Antarctica, mountain peaks like in Patagonia, fjords like in Norway all gathered in one place - the Scoresby Sund. The multi-faceted flora of Greenlandic tundra and large mammals including Seal, Musk Ox, and Polar Bear, are also great highlights of the area. From the vibrant reds and oranges of the glacier-carved terrain to the depthless blues of the icebergs that serenely float among the fjords, the overwhelming beauty of this vast fjord system fascinates any photographer who is lucky to visit this remote and rarely visited part of Greenland. The main structure of Scoresby Sund is about 110 km (approx. 68 mi) long, with a large number of islands and numerous side fjords, of which the longest extends to about 350 km from the coastline inland. Some of the fjords further inland are up to 1,450 m (4,760 ft) deep. The town of Ittoqqortoormiit is the only permanent settlement in this remote area. The name of this town with about 490 native Greenland inhabitants, the Inuit, translates into something like “those who live in big houses”. The sea ice blocks ship access to Ittoqqortoormiit for up to nine months of the year.
97 photos1; Iceland — Landscape in the Making; 155 photos; Landscape in the Making; Iceland's awe-inspiring geological features are a gift to nature photographers. The island is one of the most active volcanic regions on Earth, where almost all types of volcanic and geothermal activity can be found. Iceland has one of the most unique landscapes in the world. The fiery wrath of the island’s volcanoes mixed with the land-shaping movement of icy glaciers has created dramatic, breathtaking scenery which is every photography lover’s dream. Rift valleys and geysers, hot springs and rhyolite mountains, columnar basalt formations and lava fields, geothermal areas and lunar-like craters, multicolored glacial rivers and streams are creating endless opportunities for breathtaking images.
155 photos1; Iceland — The Coastline; 161 photos; The Coastline; Iceland’s rugged coastline, of more than 3,000 miles (4,800 km), meets the Greenland Sea on the north, the Norwegian Sea on the east, the Atlantic Ocean on the south and west, and the Denmark Strait—which separates it from Greenland on the northwest. Most Icelandic beaches are black because of the volcanic nature of their surroundings as there is old lava almost everywhere by the Icelandic shores. There are also charismatic mountain peaks, spectacular waves, icebergs melting on the black sand, beautiful basalt sea stacks, small fishing villages, and high coastal cliffs hosting bustling seabird colonies.
161 photos1; Iceland — The Land of Waterfalls; 96 photos; The Land of Waterfalls; Iceland…the land of fire and ice…and more waterfalls than you can count. All have a distinct character, and each waterfall will leave you in wonder with its shape, size, sound, or the surrounding nature around them. If you are planning a trip to Iceland, you’ve heard of Gulfoss, seen photos of Skógafoss, and you are probably planning to walk behind Seljalandsfoss. In this gallery, you will find some of my favorite Icelandic waterfalls. These range from the most popular waterfalls to off-the-beaten-path destinations. For some, you can just pop out of your car and take a photo. For others, you will have to do some hiking, but that’s an amazing experience all to itself.
96 photos1; Lake Powell Area, Glen Canyon NAR; 127 photos; Hieroglyphs of the Earth; Glen Canyon National Recreational Area. More than 1.25 million acres of incredibly diverse and beautiful land and 650 million years of the Earth's history. Lake Powell - the heart of Glen Canyon is 186 miles long and has 1,960 miles of shoreline - longer than the entire west coast of the continental United States. Lake was formed by Glen Canyon Dam and named in honor of Civil War veteran and explorer John Wesley Powell. There are 96 major canyons to explore and countless smaller tributaries. The entire area presents an unlimited potential for a landscape photographer. There are almost no roads along the shores of the Lake Powell, and the large areas of this incredible labyrinth are accessible only by air. I have organized several lend, water and aerial photo-expeditions to the area over last 12 years and have built an ultimate knowledge of its main geological features, preferable light, photogenic altitude and angles. This gallery is the result of my exploration. Enjoy!
127 photos1; Namib Skeleton Coast NP. Namibia; 108 photos; Two Oceans of the Skeleton Coast; From the Skeleton Coast in the north to the Orange River in the south, Namibia has nearly a thousand miles of coastline. For much of that distance, the windblown dunes of the Namib Desert reach right to the pounding surf of the Atlantic Ocean, creating a stark yet beautiful landscape. Shaped by the winds and largely unpopulated, Namibia’s coastal area is home to only a handful of towns and villages. Shipwrecks lie in place for decades or even centuries, slowly disintegrated by the sea and desert. In places, cape fur seals gather in huge numbers, mostly free from harassment. Dense ocean fogs originate offshore from the collision of the cold Benguela Current and warm air from the Namib Desert and often cover dunes with the cool mist. In this collection, you can take a look at Namibia’s picturesque shores and dunes, its coastal wildlife, and some evidence of the human presence.
108 photos1; Selected panoramic images - 2:1 ratio; 42 Photos;
42 Photos1; Spitzkoppe, NamibRand & Quiver Trees; 57 photos; Wonders of Namibia; The Spitzkoppe is a group of bald granite peaks or inselbergs located between Usakos and Swakopmund in the Namib desert of Namibia. The granite is more than 120 million years old and the highest outcrop rises about 1,728 meters above sea level. The peaks stand out dramatically from the flat surrounding plains.; The Namib Rand natural Reserve is one of the most beautiful places in Namibia. Brown hills, orange dunes, green-yellow plains full of wildlife, and endless sky above. The Reserve founded in 1984 by J.A.Brückner, and has more than 215,000 hectares (2,150 km2) and shares a 100 km border with Namib-Naukluft National Park to the west and the Nubib mountains to the east.; Quiver Trees prefer to grow almost exclusively atop medium-to-large dolerite rock formations, so they normally grow great distances from each other. In a small rocky pocket outside of Keetmanshoop in Namibia a large number of them grow in uncharacteristically close proximity, creating a forest-like landscape. It’s one of the only known naturally occurring sites in the world. Despite its name, the Quiver Tree (Aloidendron dichotomum) is not a true tree, but rather a species of Aloe capable of growing over 30 feet tall.
57 photos1;Cote d'Albatre/Alabaster Coast, Ertretat; 21 photo; Cote d'Albatre – The Alabaster Coast; Etretat is best known for its chalk cliffs, including three natural arches and a pointed formation called L'Aiguille or the Needle, which rises 70 metres (230 ft) above the sea. Some of the cliffs tower 90 metres (300 ft) over the Atlantic Ocean. These natural wonders are freely accessible, as is the white pebble beach. Claude Monet and other Impressionists famously painted the arches and cliffs in the nineteenth century. An Arsène Lupin story was based here while Guy du Maupassant spent most of this childhood in the town.;
21 photo1;Sentinels of Swamps; 91 photo; Sentinels of Swamps; Rising tall from dark, murky waters, the bald cypress tree is a stately symbol of the swamp. Associated with the bayou, Spanish moss, pelicans, egrets and alligators, the bald cypress is the state tree of Louisiana. Its feathery foliage, wide and buttressed base and irregular crown dominate many southeastern wetlands, and its range extends throughout the southeastern U.S. from southern Delaware to eastern Texas. Bald cypress are often found growing with another type of swamp loving tree, the tupelo. Cypress is one of the few conifers that lose its needles in the winter and the bare trunks and branches present a more haunted feeling. There is always something different to see in the swamp. Just imagine the fog-engulfed swamp with the knobby knees of bald cypresses sticking out the water and Spanish Moss, transforming the trees into an arboreal versions of mysterious shaggy creatures.
91 photo22; Atacama and Altiplano. Chile-Bolivia; 79 photos; Traveling The High Plain; Atacama is the driest desert on Earth and recent scientific studies demonstrated that the soil in the region is about 25-30 million years old. The Atacama landscape is characterized by highlands, volcanoes, salt lakes, lagoons, hot springs, strange rock formations and sand dunes. Famous natural landmarks include Licancabur Volcano, Valle de la Luna, Parque nacional del Valle de la Muerte, El Tatio Geysers and the Atacama Salt Flat. The Altiplano, meaning “high plain” in Spanish, is a high plateau in the Andes Mountains stretching across the borders of Chile, Bolivia, Argentina and Peru. Altiplano is a place of superlatives: It holds the world's highest navigable lake, Titicaca, and the largest salt flat, Salar de Uyuni. It is the second largest mountain plateau in the world, after that of Tibet. It also is home to many active volcanoes.
79 photos28; Symmetries and Beyond; 25 photos; Symmetry Studies; In architecture, symmetry is the reflection of shared forms, shapes, or angles across a central line or point called the axis. Basically, components that mirror each other across an axis are symmetrical. This is one of the oldest and most continuously used ordering principles in architecture. Symmetry helps bind various elements of a structure together into a single, unified whole. It is also commonly used to create a sense of rational order and calm logic. We can look at symmetry on many scales, from the relationship between single details, to the layout of the complete structure, and even to entire urban centers built on a symmetrical grid pattern. ©study.com
25 photos2; Iguazu – Waterfalls & Birds; 48 photos;
48 photos2; Antarctica; 63 photos; The Continent of Stark and Beautiful Desolation; Antarctica is miraculous. A remote, white continent of stark and beautiful desolation. For Nature photographers, Antarctica is, without doubt, the gold standard against which all other photography destinations are measured. Nowhere else on earth offers such incredible landscapes and wildlife in such a pristine and remote environment. Antarctica is one of the hot spots for photographing penguins, seals, and icebergs in a great landscape.
63 photos2; Lofoten Winter Magic; 60 Photos; Lofoten Winter Magic; At 68N, the Lofoten archipelago in the northern part of Norway well above the arctic circle, and at the same latitude as Greenland or the northern parts of Alaska. However, it enjoys a relatively milder climate due to the circulation of the Gulf Stream. With its postcard-looking small fishing villages nestled in fjords, dotting a very rugged coast with abrupt peaks rising directly from the ocean, the archipelago is often described as one of the most scenic parts of Norway and one of the most beautiful archipelagos in the world. In winter months you have a real feeling of being north of the arctic circle with short days and the few hours of daylight filled with one long transition from sunrise to sunset. Due to mild climate and incredible beauty, Lofoten archipelago became one of the top winter photography destinations.
60 Photos2; Miami, Pelicans & Murals; 47 photos;
47 photos2; Valparaiso Street Art; 69 photos; The walls and houses of Valpo; Aside from its historic funiculars, vibrantly painted houses, and cascading hills, Valparaiso has another claim to fame that is rapidly helping this seaside “Jewel of the Pacific” assert itself as Chile’s bohemian, counterculture city par excellence: its street art. Splashed over the walls and houses of the city, Valpo (as it’s affectionately known by residents) has amassed quite the collection of colorful and creative murals, stencils, and paintings, marking it as a top destination for street art enthusiasts, along with other street art meccas like Berlin, London, and Melbourne. As a UNESCO World Heritage Site, Valparaiso takes great pride in its history and culture, and the local government and citizens encourage and support local and visiting artists, with many businesses either requesting, bartering, or paying for their storefronts or buildings to be given an artsy touch-up.
69 photos2; “Wild Locals”; 316 photos; Wildlife in my backyard; All photos in this series were taken either in our own backyard or in a small park right next to our house in the Arlington Club Community, Wheeling, IL, USA. I took some wildlife photos in our area previously, but a major part of this collection was taken during the COVID-19 outbreak in 2020. All my workshops for 2020 were canceled and I had extra time to look outside through the patio door and take an occasional stroll in our small park. I started by installing bird feeders and was amazed by the amount and variety of birds living in our area. Chipmunks and squirrels were also common visitors to the bird feeders and I was fascinated by their ability to scale any barriers when looking for food. I also discovered that some larger mammals are present in the close proximity to our house, and had a chance to photograph raccoons, skunks, and even coywolf. Keep your shutter clicking - there is always something worth photographing!
316 photos3; Walruses & Seals; 8 photos; Every couch potato's spirit animals; Svalbard with its cool climate may not be a destination that comes to mind when people feel like going on vacation to spend some time sunbathing on a nice beach and going for a swim in the ocean. Nevertheless, the islands throughout Svalbard have their fair share of beach guests that are dead set on enjoying the sun from the best place on the beach. Say hello to the walrus!; The walrus, found year-round in Svalbard, is a picky creature when it comes to resting spots. They prefer areas with ample food and comfortable conditions. These social animals gather in packs on ice floes or land, often tightly grouped or even stacked atop each other. Their presence adds allure to the wilderness of Svalbard. An adult male is around 3,5 meters long and weighs about 1500kg, while the females grow to around 2,5 meters in length and can weigh about 900kg. One can simply say that walruses are quite bulky creatures, and if you’re lucky enough to get a glimpse of a large colony it’s truly a mighty experience of nature to see them all bunched up. They also could be very playful and enquiring. On one of the Zodiac trips, we met a gang of young walruses. They seemed to seek our attention and were very curious and animated. They were looking at us as if inviting us to play and the air was vibrating with their loud breath and splashes. They were disappearing under water and ice floes, just to bounce back on the other side of our Zodiac, even bumping into it now and then. At some point, a couple of big bulls also joined the party and were diving beneath our Zodiac. When you have 3.5 meters long animals weighing over 1.5 tons with tusks 3 feet (90 centimeters) long under your rubber boat, you may feel a bit of adrenaline. When their attention to us passed a certain point, we retreated to the ship to avoid any accidents. It was a unique and exciting experience.; Two other most common seal species found in Svalbard are the bearded seal (Erignathus barbatus) and the ringed seal (Pusa hispida). These seals are frequently observed in the waters surrounding Svalbard and along its coastlines. They have adapted to the Arctic environment and are well-suited to the icy conditions of the region. The bearded seals are characterized by their impressive beards, which consist of long, thick whiskers. They are often found in the icy waters around Svalbard, especially along the coastlines. The ringed seal, recognized by the dark rings on its light-colored fur, are emblematic of Svalbard's diverse seal population. The ringed seal is an Arctic specialist and the most abundant of the Arctic seals. Being the polar bear’s favorite prey, they are naturally nervous and shy of close approach.
8 photos4; Le Mont Saint Michel Abbey; 38 Photos;; A magical island topped by a gravity-defying medieval monastery, the Mont-Saint-Michel counts among France’s most stunning sights. For centuries one of Europe’s major pilgrimage destinations, this holy mount is now a UNESCO World Heritage Site, as is its breathtaking bay.
38 Photos8; Mono Lake, Eastern Sierra, California; 28 Photos; Spirits of Mono Lake; Located at the elevation of about 2000 meters over sea level, the Mono Lake is one of the most beautiful and unusual lakes not only in California or the USA, but perhaps on the Planet. Eerie tufa sculptures, like fairy-tail figures rise from salty waters of the lake on the backdrop of snow-caped peaks of Eastern Sierra.
28 Photos9; Kolmanskop Ghost town, Namibia; 27 Photos; Sands of Time; Kolmanskop is situated in forbidden territory of Namib Desert in Southern Namibia. It was once the home to hundreds of German miners desperately seeking their fortune in the Namibian desert. Town was built in early XIX century as a result of frantic diamond rush and was ultimately abandoned in 1954, when the diamond-field exhausted. The stately homes have been nearly swallowed by the desert dunes and heavily demolished by the wind. In 1980, the mining company De Beers, restored a number of buildings, and established and interesting museum, which has now become a major attraction for tourists and photographers.Kolmanskop is situated in forbidden territory of Namib Desert in Southern Namibia. It was once the home to hundreds of German miners desperately seeking their fortune in the Namibian desert. Town was built in early XIX century as a result of frantic diamond rush and was ultimately abandoned in 1954, when the diamond-field exhausted. The stately homes have been nearly swallowed by the desert dunes and heavily demolished by the wind. In 1980, the mining company De Beers, restored a number of buildings, and established and interesting museum, which has now become a major attraction for tourists and photographers.
27 PhotosTerra Penguinia. King and Gentoo Penguins. South Georgia, Sub-Antarctic.
54 PhotosTerra Penguinia. King Penguin Creches at Saint Andrew's Bay and Salisbury Plain. South Georgia, Sub-Antarctic.
30 PhotosSouthern Elephant Seals and Antarctic Fur Seals. South Georgia Sub-Antarctic.
30 PhotosAlbatrosses, Petrels and others. Birds and Landscapes of South Georgia Archipelago, Sub-Antarctic.
36 PhotosWildlife of Svalbard. Svalbard (Spitsbergen) Archipelago, Norway
112 PhotosPolar Bears of Svalbard. Svalbard (Spitsbergen) Archipelago, Norway
114 PhotosLandscapes of Svalbard. Svalbard (Spitsbergen) Archipelago, Norway
92 PhotosSuspended in the Air. Meteora Monasteries. Meteora, Thessaly, Greece
44 PhotosMonument Valley Tribal Park - Agathla Peak - El Capitan, Owl Rock, Church Rock
54 PhotosHottest, Driest, Lowest, Death Valley National Park, California, USA
54 PhotosAlabama Hills and Eastern Sierra. Inyo County, Lone Pine, California, USA
36 PhotosArches National Park, Utah, USA
36 PhotosUpper Antelope Canyon. The Light Play. Arizona, USA
18 PhotosLower Antelope Canyon. The Light Show. Arizona, USA
18 PhotosThe Wave. North Coyote Buttes, Paria Canyon, Vermilion Cliffs Wilderness Area, Arizona, USA
18 PhotosBryce Canyon National Park, Utah, USA
59 PhotosPalouse, Eastern Washington, American Tuscany, USA
36 PhotosCanadian Rocky Mountains, Landscape and Nature Photography, British Columbia and Alberta, Canada
54 PhotosOlympic National Park, Washington, USA
36 PhotosStarved Rock State Park, Illinois, USA
7 PhotosSand Dance Series
18 PhotosSandbox for Men
18 PhotosTango. Buenos Aires, Argentina
36 PhotosBuenos Aires, Argentina. People and Places
54 PhotosMurals & Walls. Buenos Aires, Argentina
18 PhotosCats of Recoleta. La Recoleta Cemetery, Buenos Aires, Argentina
18 Photos