one year. Associated words: annual, annually.
on horseback. mounted.
onion, n. (that does not bottom out) scallion, eschallot, shallot. Associated words: alliaceous, cepaceous, leek, allium, garlic.
onlooker, n. looker-on, [bystander], [spectator], [beholder], [witness], eye-witness.
on tap. broached, abroached.
on the sly. (Collog.) [sly], covertly, furtively, clandestinely, surreptitiously, secretly.
ooze, v. [filter], exude, [percolate], transude, [seep].
oozing, n. [percolation], [filtering], exudation, [distillation], transudation.
opalescence, n. iridescence.
opalescent, a. [iridescent].