The on-loan Tottenham midfielder was among the goal scorers and highlighted David James' performance between the posts for Pompey.
"It was fantastic for the players, the fans and all the staff at the club," O'Hara told the club's website.
"To come here in front of a hostile crowd and win 4-1 is absolutely fantastic. I've played in derbies before, like Tottenham v Arsenal, but that was something else.
"They were up for it and were on top of us in the first half. Their longs throws were causing us problems and we had to ride our luck.
"But David James was fantastic for us and three world-class saves kept us in the game."