HackerVoca

operative : effective, efficient, effectual
opponent : adversary, antagonist, contestant, enemy, foe
opportune : timely, seasonable
opportunity : chance, occasion

oppose verb [transitive]

1 to disagree with something such as a plan or idea and try to prevent it from happening or succeeding: Congress is continuing to oppose the President's healthcare budget. | be opposed to sth: Most of us are opposed to the death penalty.

2 to fight or compete against another person or group in a battle, competition, or election: He is opposed by two other candidates.