The former England and Manchester United player has been ruled out since he sustained a knee ligament injury during a pre-season trip to South Korea.
The 33-year-old suffered a setback in his recovery in October and now looks set to miss Sunderland's entire campaign, which continues with a trip to QPR in the Barclays Premier League on Sunday.
"It's been a long time out for Wes, who has been beset with injuries," O'Neill told the club's official website.
"He looked as if he was just settling at the time I came in, but hurt himself in January last year and we've not really seen him back since.
"He had a setback out in Korea but it looked like he was making progress, but for Wes it's been a case of two steps forward and three back.
"Chances are we probably won't see him between now and the end of the season, but perhaps it'll be one he wants to forget and start again next season."
Brown joined Sunderland from Man Utd in July 2011 on a four-year-deal for an undisclosed fee but saw his first season for the club cut short when he was injured during the FA Cup third round tie with Middlesbrough in January 2012.
Following the injury to his knee in pre-season training for this campaign, the veteran defender has seen a specialist in London for scans and treatment.
O'Neill confirmed Brown has been back in training but is unlikely to feature in any of his side's remaining fixtures.
"He's joined in training on a few days and then has to take some time out again," O'Neill said.
"At the moment, however, I think he's reversing that trend - it's one little step at a time at the moment."