United take on O’Neill’s side at Vale Park on Wednesday and knows that his side have a big job on their hands to prevent Rooney from hitting his 22nd Premiership goal of the season.
"It is Manchester United in town and at the moment they are playing particularly strongly," said O'Neill.
"Rooney is in fantastic form. He has been an exceptional player for some time. He is probably playing the best he has ever played at the moment, scoring goals as well, which gives you a big boost of confidence.
"He has been a real top quality player for the last four or five years. Has he stepped up after the loss of Carlos Tevez and Cristiano Ronaldo? It seems he has taken on even more responsibility and he is thriving on it.
"Yet last season, when United wanted to play him wide right or wide left to help maybe stop the opposition from playing, Rooney was the first one to volunteer. Now he (Sir Alex Ferguson) has got him playing basically as a centre-forward, either with Berbatov or on his own. He is up to the challenge and he is just a terrific footballer."