Sunday's result extends the Black Cats' winless run to seven games and leaves them four points above the relegation zone.
O'Neill has conceded that the players are struggling to deal with the situation.
"The pressure has got to the players," he told The Sun.
"When you are on the run we are at the moment then confidence is a wee bit low. We should have made the extra man count - it is frustrating.
"The whole league's been tough for us. My worry is we have players on international duty and won't see them for some time. I hope they come back in one piece."
Sunderland face Manchester United on March 30.