O'Brien says that the rule of the party is forever. However, the appendix appears to be written in a future time when there is no party. Is this a contradiction in the book?

Yes it is a contraction in the book because it was written when there was no party. If O'Brain had written it when party was in existence it would not have been a contradiction but because it was written prior to the existence of party it is a contractions in the book.

The party that was later held was for a purpose and it is to seek power.