To be party to something is to participate in or be involved in it. The antonym, or opposite, of being party to would be to not have any involvement or participation in the matter. This could be described as being unaware, ignorant, or uninvolved. For example, if there was a legal case and someone asked if you were party to it, responding with "no" would indicate that you are not involved in any way. Similarly, if there was a conflict between two individuals and you were not aware of the details, you would not be party to the dispute. In essence, the antonym for being party to would be to have no connection or involvement with the matter at hand.