Willow oak is a medium to large tree with a dense, pyramidal crown (becoming more rounded with age) and straight, clear trunk.
Leaves are alternate, simple, 2–5 inches long, to 1 inch wide, shaped like willow leaves, narrow, gradually tapering at both ends, thick; margin entire, bristle-tipped. Leaves turn pale yellow in fall.
Bark is smooth, light reddish-brown on young trees; dark gray with rough, irregular, scale-covered plates and shallow grooves when older.
Twigs are slender, reddish-brown and hairy; gray and smooth with age.
Flowers April–May, in catkins.
Fruits September–October, acorns solitary or in pairs; nut brown with dark stripes, rounded, about ½ inch long; cup covering about ¼ of the nut, saucer-shaped, shallow; scales small, flattened, greenish- to reddish-brown, finely hairy; seed bitter; ripen in autumn of the second year.
Similar species: Shingle oak (Q. imbricaria) has wider leaves and deeper acorn cups; it is also more widely distributed in Missouri. Willow oak is a type of oak and is not at all a willow; it was named for its willow-like leaves.