O’Neill was left seething after Joe Lolley’s second half goal was ruled out for a foul on keeper Declan Rudd.
It was the second game in a row the Forest supremo has been left feeling cheated after Dwight Gayle won a late penalty by diving in their 2-2 draw with West Brom.
“The keeper spilt the ball and it was there to fight for,” he said.
“We have been robbed of all three points again.
“That is four points in two games.
“It should have counted and I told the ref he should look at it again.
“These are the points that you cannot get back.
“At the start of the season you can ignore this and hope it levels itself out.
“That is a lot of points to lose out on at this stage of the season.”
Forest’s play-off hopes suffered a dent after they were held.
The visitors enjoyed the better of the game as they watched a number of first half chances go begging.
Alex Milosevic, Joe Lolley and Yohan Beanlouane all went close before Lewis Grabban and Lolley (for a foul in the build-up play) had goals ruled out.
Despite their lacklustre display, Preston could have won it with Brad Potts having two good chances to give his side all the points.
But both side’s lacked real quality as the game aimlessly drifted towards stalemate in the final 15 minutes.
O’Neill added: “We had the better chances in the first half and I thought we were going to score.
“We started slowly and had to defend strongly. Preston had got confidence from beating Norwich in midweek
“As we got into the game we created decent chances and deserved to get all three points.
“Overall I was pleased with the effort of the side.
“They put in an big shift on Tuesday night (at West Bromwich Albion) and to come here and do it again was very good.
“Preston are a good side and in good form, and I am pleased with the team.”