OBCORDATE: Inversely heart-shaped. The wider end and its deep notch at the top instead of the bottom. See Cordate to distinguish.
OBDELTOID: Inversely deltoid. Triangular, with the apex downward. See Deltoid to distinguish.
OBDIPLOSTEMONOUS: Having the stamens in two whorls, those of the outer whorl being opposite the petals.
OBDIPLOSTEMONY: The condition of being obdiplopstemonous.
OBLANCEOLATE: Inversely lanceolate; shaped like a lance-point reversed, with the tapering point attached to the leafstalk.
OBLIGATE: Restricted to a particular way of life.
OBLIQUE: Slanting with unequal sides.
OBLONG: Longer than broad, and mostly parallel-sided, such as an oblong leaf.
OBOVATE: Inversely ovate; having the broad end upward or toward the apex, as in many leaves. See ovate to distinguish.
OBROTUND: Approaching a round form.
OBTUSE: Blunt or rounded at the extremity, as an obtuse leaf, sepal, or petal.