Plants shrubs or trees (lianas elsewhere), often incompletely monoecious or dioecious (with at least a few perfect flowers among the pistillate and/or staminate ones. Branches sometimes angled or ridged, at least when young. Leaves opposite (rarely subopposite), sessile or petiolate. Stipules absent. Leaf blades simple to ternately or pinnately compound. Inflorescences terminal and/or axillary, variously dense to loose clusters, racemes, or panicles, rarely reduced to solitary flowers, the axes not coiled at the tip, the inflorescence branch points often with small, leaflike or scalelike bracts. Flowers actinomorphic, hypogynous, perfect or imperfect, lacking bracts (except in Jasminum). Calyces small or minute, less commonly absent, when present shallowly to more commonly deeply 4–6-lobed, sometimes merely toothed or unlobed and truncate, usually persistent at fruiting (shed early in some Fraxinus species). Corollas absent or more commonly present and shallowly to deeply 4–6-lobed. Stamens 2 (rarely 1 or 3 in Forestiera or 3–5 elsewhere), the filaments attached at or above the base of the corolla tube, the anthers not exserted or short-exserted, attached at their bases, dehiscing by a pair of opposite, longitudinal slits, sometimes with a small, toothlike, sterile, terminal extension, usually yellow. Pistil 1 per flower, of 2 fused carpels. Ovary superior, sometimes with a small nectar disc surrounding the base, 2-locular, with usually 2 ovules per locule (sometimes numerous in Forsythia), the placentation axile. Style 1 (rarely absent), either unbranched with a single, capitate stigma or more commonly forked at the tip with a pair of small stigmas, usually withered or absent at fruiting. Fruits drupes, berries, samaras, or longitudinally dehiscent capsules, variously shaped, usually glabrous. Seeds 1(–4) per fruit (usually 2 in Syringa; numerous in Forsythia). About 25 genera, about 600 species, nearly worldwide.

The family Oleaceae contains a number of economically important species. Several genera are cultivated as ornamentals, including at least some species in all of those treated in the present flora, as well as Osmanthus Lour. (osmanthus, fragrant olive) and Noronhia Stadman ex Thouars (Madagascar olive). Fraxinus contains commercially important timber trees whose wood is used in furniture, flooring, veneers, baseball bats, hockey sticks, canoe paddles, other implement handles, and handcrafts. Perhaps the most important species economically is the European olive (Olea europaea L.), which is cultivated as an ornamental in warmer climates, but is also the source of olives and olive oil. Some of the introduced species, principally of Ligustrum, are considered invasive exotics, especially in the southeastern states.