As adjectives the difference between deponent and opponent
is that
deponent is having passive form (that is, conjugating like the passive voice), but an active meaning (such verbs, originally reflexive, are considered to have
laid aside their passive meanings) while
opponent is situated in front; opposite; hence, opposing; adverse; antagonistic.
As nouns the difference between deponent and opponent
is that
deponent is (legal) a witness; especially one who gives information under oath, in a deposition concerning facts known to him or her while
opponent is an individual or group who is a rival of another.