Ant. Closed, barred, inaccessible, unavailable, reserved, shut, secreted.
Opening. Aperture, hole, space, breach, gap, fissure; start, inauguration, commencement, initiation, beginning; opportunity.
Ant. Obstruction, stop-gap, termination, enclosure, close, end, conclusion, inopportunity, unseasonableness.
Operation. Agency, action, exercise, production, influence, force, performance, result, procedure, effect, execution.
Ant. Failure, uselessness, futility, ineffectiveness, inefficiency, powerlessness, inaction, cessation, rest.
Opinion. Persuasion, idea, sentiment, view, conviction, judgment, notion, impression, estimation.
Ant. Evidence, argument, inquiry, speculation, investigation, pleading.
Opponent. Antagonist, foe, adversary, enemy, rival.
Ant. Helper, assistant, accomplice, ally.
Opportune. Timely, seasonable, convenient, ready, suitable, auspicious, meet, appropriate.