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.

opaque, a. intransparent; (Colloq.) obscure, unintelligible.

open, a. accessible, clear, unobstructed, unrestricted; ajar, unlocked; extended, expanded, spread, gaping, yawning, unfolded, dehiscent; frank, unreserved, candid, ingenuous, guileless, overt, undisguised; generous, liberal, bounteous, open-handed; revealed, patent, manifest; unsettled, undetermined, debatable, undecided, controvertible; mild, clement; free, disengaged, unappropriated, unemployed.

open, v. spread, expand, unfold, evolve; reveal, disclose; unclose, unlock, unfasten; begin, commence; dehisce.

open-air, a. outdoor, al-fresco.

opening, a. beginning, introductory, initiatory, preliminary.

opening, n. aperture, fissure, gap, orifice, rift, chasm, hiatus, rent; beginning, introduction, outset; vacancy.

openly, adv. publicly, unreservedly, candidly.

open-mouthed, a. gaping, yawning; greedy, clamorous, ravenous, rapacious, eager.

opiate, n. narcotic, anodyne, sedative, dormitive, soporific.

opiate, a. somniferous, narcotic, anodyne, sedative, soporiferous, dormitive.

opinion, n. belief, judgment, impression; decision, ruling, verdict, sentence.

opium, n.  Associated Words:  narcotic, opiate, chandoo, thebaine, narcotine, codeine, dope, meconism, meconology, meconophagism, meconophagist, opiophagism.

opponent, n. adversary, antagonist, rival, competitor.

opportune, a. timely, seasonable, apropos, felicitous, appropriate.

oppose, v. combat, resist, confront, withstand, oppugn, impugn, contend, antagonize, contravene, discountenance, gainsay, contradict.  Antonyms:  yield, acquiesce, concur, agree.

opposer, n. opponent, antagonist.

opposing, a. conflicting, antagonistic, contending, opposite, adverse.

opposite, n. antithesis, counterpart, antipodes.

opposite, a. contrary, adverse, diametrical, antagonistic, inverse, reverse, contradictory, counter, incompatible, antithetic, antipodal, inconsistent.

opposition, n. resistance, counteraction, hostility, repulse, rebuff, recalcitration, counterview, antagonism, contradiction, obstacle, impediment, obstruction.  Antonyms:  acquiescence, non-resistance, concurrence, agreement.

oppress, v. burden, overwhelm, aggrieve, tyrannize, persecute, overpower.

oppression, n. tyranny, persecution, extortion, hardship, severity.

oppressive, a. rigorous, tyrannical, extortionate, burdensome.