And while O'Gara praised Quins for their historic efforts, the ERC Dream Team member insisted he and his team-mates had fallen below their usual high standards.
"Harlequins was a very poor day," said O'Gara, who was a Heineken Cup winner in 2006 and 2008.
"Harlequins are obviously a good team and I am not taking away from them for one minute, but you would expect us firing on all cylinders to beat Harlequins. But expecting and doing are two different things, and we didn't do it.
"We struggled in all areas, our execution was poor and, at this level, small margins make a big difference. We didn't give ourselves a chance."
But while O'Gara could not hide his disappointment at the manner of his side's European exit, he remains confident that Munster can bounce back, first with a Magners League title this month and then with a third continental crown next term.
Munster have experienced their fair share of heartbreak in Europe, narrowly missing out on Heineken Cup glory in 2000 and 2002 and falling at the semi-final stages on five previous occasions.
O'Gara and co have repeatedly bounced back and he believes Saturday's defeat could help a younger generation of Munster stars develop the same resolve as Munster look to become the Kings f Europe once more in 2012.
"But the great thing about sport, and rugby, and the Munster team, is you get another opportunity," added O'Gara.
"There's an awful lot of the senior players that were young fellas once upon a time and they've had bad days before and if the right lessons were taken out of last weekend by the younger players it will accelerate their development hugely.
"But we do need fellas coming in and not looking just to get on the team, but coming in and trying to win European Cups.
"I think the ambition is huge and that's something the team needs."