Leinster started off superbly, racing into a 10-0 lead, but faded as Ulster boosted their chances of a home semi-final with 26 unanswered points.
And while O'Connor was reluctant to blame last week's defeat in Toulon for the loss, he does believe it took its toll on his players.
"We were working pretty hard, but we were a little bit flat I think," he told RTE Sport.
"The emotion that went into last week, and the disappointment that was left over, probably hurt us and once the game was out of our grasp we toiled hard but we just didn't have that edge to us.
"And to come away up here, even off the back of really good preparation, is always hard.
"Ulster were up for it. There was a lot at stake from their perspective, and they were deserving winners.
"We haven't raised our intensity enough. We probably stayed at the intensity where we've been over the last couple of seasons, and it [the rest of the season and the Summer break] will give us the opportunity to review it and have a look at it and make sure that we are better next time around.
"The dynamic of meritocracy has changed. Teams are playing for every point. There's a growth in the competitive edge to every game, which is really positive for the GUINNESS PRO12."
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Guinness PRO12 Leinster News
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