Two second half goals from Marvin Sordell sealed victory for Dougie Freedman's men and left a frustrated O'Neill admitting his side only had themselves to blame for their "rather too meek" performance.
O'Neill said: "It was really disappointing following Saturday's excellent display, and we need to start getting some consistency to our game.
"We're going to have to get it. This is a big club and we shouldn't be spending 40 years without winning something, we really shouldn't.
"We didn't take anybody lightly here tonight, we just got beaten because we didn't create enough chances in the second half.
"I know some people field weakened sides but we didn't, we tried to win tonight and we weren't able to do it, and that was very disappointing."