On-loan Tottenham midfielder O'Hara handed Pompey a first-half lead before Rovers came storming back.
O'Hara told pfctv.com: "They brought on Roberts at half-time and that changed the game. They really put us under pressure and were bombing it into the penalty area and we couldn't deal with it.
"We were sitting there at half-time thinking we could be out of the bottom three going into the international break.
"But now we go away rooted to the bottom with 15 days to think about it before our next game. But we've now got to go back into training and pick ourselves up."