Ireland added a penalty try and a late effort from replacement hooker Jerry Flannery to bolster their 20-3 half-time buffer, but converted tries from Namibia's Jacques Nieuwenhuis and Piet van Zyl saw the men in green embarrassingly outscored in the second half.
Speaking straight after the game, O'Sullivan said: "It was probably our worst performance for a long time. I thought we would have nerves at the start and make some mistakes, but we actually started well and put some scores on the board.
"As the game went on we got looser and looser, lost our shape, made a lot of individual errors and the crowd got behind Namibia. They upped their game and played really well.
"It was a very poor performance from us, very disappointing all-round. We've got to take a long look at ourselves because we've got another game next week and if we play like that against Georgia, it could be a worse night."
Forgettable as it was, there were still some positives for Ireland to cling onto - a good first quarter and gaining a bonus point which puts them top of the Pool D standings.
But O'Sullivan warned Ireland will have to dramatically lift their game if they are to have any hope of making the knock-out stages and living up to their own expectations and those of the travelling support.
"I was happy after the first quarter, we got the scores and we were ticking over. But we went downhill after that, we lost our shape before half-time.
"At half-time I spoke about doing the simple things well, but we went out in the second half and just compounded that by making more silly errors. Namibia thrived on our mistakes, got the crowd behind them and we had a game on our hands," admitted the Corkman.
"The only thing is we got the win and the bonus point. We didn't get the score we wanted - we'd hope to put more poins on the board - whether that has an impact on the pool that remains to be seen.
"Performance is the main thing. If we keep performing like this we're not going anyhere. We've another game next week, we'll circle the wagons, take it on from there. We've no option to improve, and improve dramatically."
That improvement has to start against Georgia at the same venue next Saturday. Looking ahead to that second pool game, O'Sullivan added: "We haven't seen them play yet (in the tournament) but we'd expect they will put up a bigger challenge than Namibia. In all fairness to Namibia, they are ranked higher than them in the world so we'd expect that. They're a very physical team.
"If we go out and reproduce tonight's performance against Georgia, we'll be in trouble."