The word oppositor does not appear in most American English dictionaries, but it does appear in the Oxford Living Dictionary. Please, is it obsolete, archaic, or only in British usage?
It is an archaic uncommon literary term; as you can see from Ngram oppositor has never been common usage. Opponent is the more commonly used term. It appears that John Florio (of Italian origin) used the term first.
- Late 16th century; earliest use found in John Florio (1553–1625), author and teacher of languages. From classical Latin opposit-, past participial stem of oppōnere to set against + -or, probably after post-classical Latin oppositor, Italian oppositore, or Middle French, French † oppositeur.