The Dutch striker scored one goal and provided another for Wayne Rooney in Manchester United's 3-1 win over the Black Cats on Saturday and his output was likened to that of Barcelona great Lionel Messi who has so much influence on his teammates.
"(van Persie) can get goals out of very little. It looks like nothing is happening, you have players in the area, but all of a sudden he has opened up a gap," O'Neill told reporters.
"United have paid close on £30m for a player (£24m) who had just one year left on his contract, so that might tell you how valuable they perceive him. But it's not just the perception, the reality is that he has been fantastic."