Delph has made only a handful of appearances since moving from Leeds in a £6million summer deal.
But he produced an impressive show against Gus Poyet's side and broke his scoring duck for the club during the second period.
O'Neill said: "Fabian was excellent in the last round against Blackburn and he again showed what a capable footballer he is. He has got a really good future ahead of him.
"He found it frustrating when he came here after playing over 100 successive games for Leeds.
"He started against Wigan on the opening day but I knew he would play a couple of games, then go out of the side, and then maybe play another couple as he got used to the Premier League.
"That can be frustrating and in addition a couple of our midfielders in James Milner and Stiliyan Petrov have been playing very well.
"It was tough for him to break through, but he will break through. He has a very good future in front of him."