However, the Dragons roared back as Jack Dixon's score and James Benjamin's try double gave the hosts the most dramatic of 25-22 triumphs.
Leinster now lie eight points behind Ospreys in the hunt for the final play-off place with just three games remaining, meaning they must rely on other results going their way to book a top-four spot.
They will look to bounce back in a huge European semi-final against Toulon next weekend but O'Connor is under no illusions about the stark reality of their GUINNESS PRO12 situation.
"Qualifying for the play-offs is now almost definitely out of our hands," explained the Australian.
"We've got three games remaining and will try to get as many points as we can out of them but now we have to hope other teams do us a favour.
"We have to find a balance between the league and Europe but that's the dynamic at this time of the season and something we understand at Leinster. You have to make adjustments and can't make excuses.
"We were relatively pleased with the first 50 minutes and pretty much in control of the scoreboard. But then we made errors and lost big moments.
"The yellow card [to try-scorer Ben Te'o] was massive. We wanted players to put their hands up for selection. Some took their opportunity and others regressed."
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