With little over 48 hours between games and a squad seriously depleted by injuries, the DW Stadium performance exceeded O'Neill's wildest expectations.
O'Neill said: "I thought it was the best result since I arrived. Manchester City was a great win for us, to score in the last minute, it seemed as if we'd played three games, particularly in the last 20 minutes of the game."
He added: "We've had a day less to prepare (than Wigan). I accept the fact the adrenaline's still flying with us but to come here and win today was a fantastic performance. I just don't know where they got the energy and will to keep going from."
The victory took Sunderland's haul since O'Neill took charge six games ago to 13 and left Wigan rooted in the bottom three.
The Latics' 2-1 victory at the Stadium of Light in November was Steve Bruce's last in charge of the Black Cats and also kicked off a more promising run of results that was brought to a crashing halt tonight.
Manager Roberto Martinez was once again left bemoaning the rough end of a refereeing decision after Mike Dean ruled Ben Watson had felled Nicklas Bendtner for Craig Gardner's opener.
The Spaniard said: "The first goal was going to be vital. In the situation that Sunderland were, they had an outstanding result two days ago and we knew we were going to play a side with great confidence but probably who were going to sit deep and defend.
"It was going to be a matter of creating enough chances to open them up and score the first goal. I was delighted that we were patient in the first half, we opened them up two or three times and hit the post twice.
"They were key moments and then Sunderland take the lead with a world-class strike from a free-kick that was non-existent and from then on it became a very hard job. We wanted to open the game, we took risks and Sunderland took advantage on the counter."