The Blues believed that they were denied four penalties as Barca ran out winners on away goals after a 1-1 second-leg draw at Stamford Bridge.
The Norwegian official has now admitted in an interview with Spanish newspaper Marca that he was not proud of his performance on the night.
"It was not my best day, really," Ovrebo said. "But those mistakes can be committed by a referee... and sometimes a player or a coach. Some days you're not at the level you should be. I can't be proud of that performance.
"They were handball situations. I judged them on the pitch and I think it is not interesting to know what I think of those actions once judged, although I understand that people think differently to the decisions I made at the time. That discussion will continue eternally."
Chelsea went on to beat Barcelona en route to their first ever Champions League triumph in 2012, and face the Spanish giants in the last 16 this time around with the first leg at Stamford Bridge on Tuesday night.