It is best to try identifying oak trees in the spring, summer and fall, since their different leaves are a key factor in accurate identification, and these are deciduous trees that lose their leaves in the winter. However, you may be able to identify a few of the oaks in early winter, if only for the fact that they keep their leaves longer. There are several main types of oaks to be found commonly in North America and Europe, which will be addressed here, but keep in mind there also are some other, rarer oaks that now are mostly found in arboretums, botanical reserves and other specialized locations. While oaks are commonly divided into red and white oaks, there are more than 400 varieties worldwide, and identification is rarely simplistic.
Look at the bark to be sure it is an oak. Most oak trees' bark and structure are similar; they usually have rough, corrugated, flaky bark when mature. Oak sapling bark is similar, though not as rough. So you need to rely on leaves and acorns to identify most oaks.
Look at the shape of the leaves. This will tell you whether it is in the red or white oak subtype, or some other variety. The characteristic "oak" shape of the leaf is long, with lobed or serrated edges. Red oaks have more pointed or "bristle" tips, while white oaks have rounded-edge lobes. If it does not fit these shapes, it may be a holly oak or a live oak. These both have rounded, shiny, dark evergreen type leaves. Holly oak leaves are longer and thinner than live oak leaves, with lower leaves showing spiky edges, and can be found in the Mediterranean, across Europe and North America. The live oak species can also be identified by its acorns, which are long, red and thin, unusual among oaks. The live oak is found in the southern U.S. and northern South America. Other oval-leaved oaks are the shingle and willow oak.
Factor in the geographical location of the tree. Some tree varieties are only found in certain areas like nuttail, scarlet, turkey and northern pin oak are usually found in North America.
Check the soil. Oaks prefer heavy and clay soils, but can be separated by whether they are growing in bottomland or wet soil, or higher, hilly locations. Foresters describe the latter groups as upland oaks, and they include Northern and Southern red oaks, white, black, scarlet, blackjack, shingle, chestnut, post, and Chinquapin oaks. Bottomland oaks include pin, water, willow, nuttail, cherrybark, shumard, bur, overcup, swamp white, and swamp chestnut oaks.
Look at the details of the leaves. Common oak leaves are dull green, with four or five rounded lobes on each side of the leaf. They don't have much stem, and grow in clusters. Sessile oak leaves are wider at the tip than the stem, and have much longer stems. Northern pin oak leaves have very distinct lobes with deep angled, pointed ends. Nuttail oak leaves also have larger, more pointed lobes than common oaks, and scarlet oaks have thinner, more angled leaves. Blackjack oak leaves look a lot like the clubs in a deck of cards, while post oak leaves have squared-off lobes. The turkey oak has deep green, glossy and very narrow, saw-toothed leaves.
Take a few leaves, acorns and recent photos of tree with you to show an arborist or botanist. Many cities, counties and state university extensions have arborists or other tree specialists who may be able to help you to identify the tree. However, be judicious in sample collecting. Don't strip bark off trees and don't break branches or twigs unnecessarily; use fallen leaves and acorns if possible.