Van Persie has already scored 15 goals for United after he joined them at the start of the season from Arsenal in a deal worth 22 million pounds, and O'Neill believes the Dutch forward is paying back every single penny spent on him.
"With players of the calibre of Van Persie in your side, you always have a chance of winning football matches. Would, for instance, Barcelona have the same effect if Lionel Messi didn't play?" the Sun quoted O'Neill, as saying.
"The number of goals Messi has scored is incredible. They are a very talented team but without him, you don't know, because Messi is their talisman," he added.
"Honestly, Van Persie can get goals out of very little. It looks like nothing is happening and you have players there in the area. But all of a sudden he has opened up a gap," he said.
O'Neill added: "You always felt that United would give themselves a big edge by getting Van Persie. He was a great purchase. It means Manchester United are capable of scoring goals because Wayne Rooney is a top quality player as well."
"We are making judgments before Christmas here - but if Van Persie stays fit, that will make a massive difference to United's challenge. They have a great chance of winning the league," he said.
"United have paid close on 30million pounds for a player who had just one year left on his contract, so that might tell you how valuable they perceive him to be. But it's not just the perception, the reality is that he has been fantastic so far," he further added. (ANI )