The phrase "was a party to" refers to an active involvement or participation in an event, action or decision. Its antonyms could be "was not involved", "was not present", "was not participating", "was not included". Other antonyms to "was a party to" could be "was out of the loop", "was not privy to", "was not consulted", or "was not engaged". These antonyms suggest a lack of involvement or participation in something, as opposed to being actively involved. Depending on the context, these antonyms can be used to convey different meanings and highlight the degree of participation or influence one had in a particular situation.