France won the try count four-two while fly-half Frederic Michalak booted 17 points as Les Bleus ran out comprehensive winners, but O'Sullivan felt the visitors deserved to finish with greater reward following a brave performance.
"We made some errors and were punished cruelly for them - they had four opportunities and took them all. The best team won but 35-17 was not a fair reflection of the way we played," he said.
"It was hard work out there. We finished the game on their line and were unlucky not to put another try past them. The final try count was reasonable, but it would have fairer had it been four-three.
"We started well and got ourselves into good field positions. We pinned them back and prevented them for building any momentum, which was exactly what we had to do. But we didn't score the try we needed and then they broke out.
"They deserved to lead at half time and we knew the second half would be crucial, but they dealt the killer blow with those two tries. I felt our errors contributed to those tries more than the French did.
"Those scores could have knocked the guts out of us but we kept battling. We showed composure at a point when in the past Ireland teams would have caved in.
"I'm pleased about that because the Stade de France is a not the place to be when things are going wrong."