At the right you see a flowering branch of a Black Oak, Quercus velutina, from a tree in southern Mississippi. It was scanned in mid March just as the tree's leaves and flowers were appearing. The leaves in the picture are only about one-fifth their summer size.
If you don't know what a calyx or a stamen is, you can review these items on our Standard Blossom Page. The yellow, wormlike items in the picture's lower, left corner are catkins, more technically known as aments. Catkins are clusters, or inflorescences, of male flowers. Each of the "bumps" on the catkins is a male flower consisting of a bract (a highly modified leaf), a lobed calyx and some pollen-producing stamens. Once the stamens have released their pollen into the air, the entire catkin will fall from the tree. Maybe you've seen thousands of such spent catkins littering a sidewalk beneath an oak tree early in the spring. Other trees producing catkins include willows and birch.
In North America you can "key to species level" your unknown oaks at the Flora of North America website. Go to the key near the bottom of the page you arrive at when you click the above link, figure out your unknown oak's "section," click on that section, and then key out your unknown to species level using the resulting key. These are technical keys using technical terms, but they are very useful. Often you'll need acorns during the keying process.
On a flowering oak twig you have to look close to see the female flowers -- the future acorns. The inset in the picture's lower right corner shows a much-magnified female flower. Actually, mainly you just see the reddish 3-lobed stigma. Below the stigma there's an egg-shaped ovary camouflaged so well that it blends with the fuzzy petiole beneath it, and the fuzzy stem just to its left. Since these female flowers appear where you might expect a bud to be most people overlook them, thinking they are seeing buds. However, a bud would never be topped with a 3-lobed stigma!
There are dozens of species of oaks and they can all be divided into two great groups, depending on how long it takes for its female flowers to develop into acorns. Acorns in the White Oak Group mature the year they appear, and the acorn's kernel is often sweetish and edible. The Chestnut Oaks belong to this group.
Acorns in the Red and Black Oak Group mature in their second year and often their kernel is so bitter that a human would have a hard time eating it.