Steven Fletcher's strike three minutes before half-time proved the difference on the south coast as the Black Cats held out for a 1-0 win in difficult conditions at St Mary's.
"In the last three games, we've been able to win two games against sides beside us at this moment," he said, referring to the 3-0 defeat of Reading and Saturday's victory.
The Black Cats now face tough matches in quick succession against Manchester City, Tottenham and Liverpool.
"It has been a tough old season right from the start and I just feel that win today, obviously before Christmas and with the programme we have coming up in the next three games, is massive for us," added O'Neill. "We got a bit of confidence from that, although we have a fair few games to go."
Southampton manager Nigel Adkins, meanwhile, was left frustrated as his side failed to get going after a two-week break.
"It just didn't happen for us today," he said. "For me, it was uncharacteristically flat for whatever reason.
"Obviously I will digest that and with the goal we conceded just before half-time, there are two or three people that could have done better in stopping the goal.
"That has obviously given Sunderland something to hang onto in the second half and, to give them credit, they defended well.
"We tried to change things around early to give ourselves an opportunity to increase the tempo, find a way forward, stretch the game to create openings, but unfortunately we haven't scored any goals."