oar oasis oath obedient object objection objective obligation obligatory oblige obliging oblique oblivious oblong obnoxious obscene obscure observance observatory observe obsess obsession obsolete obstinate obstruct obvious occasion occupy occur occurrence odd odds odds and ends odour odyssey offence offend offer off-hand offset offshore offspring omen ominous omit on account to on and off on and on on behalf of on board on condition that on edge on end on good terms with sb. on hand on no account on the air on time once once in a while once more oncoming one one another ongoing onlooker onset opaque open sb's eyes to opening opinion opponent opportunity oppose opposite opt optical optimism optimum option optional orbit orchard orchestra ordeal orderly ore organ organism oriental orientate orientate oneself origin original originate ornament orthodox oscillate ostensible ostentation oust out of line with out of stock outbreak outcast outcome outcry outdated outdo outfit outlaw outlet outline outlying output outrageous outright outset outskirts outspoken outstrip outweigh oval ovation overall overdue overflow overhaul overhear overlap overlook override overrun oversee overt overtake overthrow overture overwhelm overwrought owe ownership oxide