Sunderland recovered from conceding a goal inside the opening 10 minutes and Craig Gardner's 34th minute sending off to salvage a 1-1 draw thanks to O'Shea's 63rd minute finish.
The point lifted Sunderland two places up the Premier League table, but Paolo Di Canio's side are only three points ahead of third-bottom Wigan Athletic, who face Swansea City in their game in hand on Tuesday night.
"I hope it's very valuable," O'Shea told Sky Sports about the importance of his goal. "We have to make sure we focus on Southampton now.
"It's a massive game again at home. We've got knocks and suspensions, and we'll dust ourselves down and go again."
The Black Cats face Southampton at the Stadium of Light on Sunday before travelling to Tottenham Hotspur on the final day and on-loan full-back Danny Rose hopes another win will secure safety.
Rose said: "The lads dug in deep and we hope it will be a valuable point.
"I think it depends on tomorrow with Wigan. These two home games against Stoke and Southampton, we felt we could get three points in both.
"We hope now to recover well for Sunday, try and win the game and I think we'll be safe."
When asked what had been key to Sunderland coming back for a point, Rose added: "I think it was patience.
"We knew we would get chances, but with 10 men it was always going to be hard when they had two banks of four behind the ball.
"We had to stay patient and now we wait for Wigan tomorrow, and hope they slip up."