The Irish playmaker appeared to stamp on Joe Garner after Carlisle's second goal-scorer had been involved in a tackle which left O'Callaghan's team-mate Joel Lynch on the ground. The visitors failed to make their numerical advantage count and Forster equalised with six minutes left.
Bas Savage was the provider for his former Reading team-mate with a cross which bounced once before Forster swivelled to fire his seventh goal in as many games and ninth in total. It was also the fourth time this season that Forster, a £75,000 summer buy from Hull City, has netted twice in the same game.
Carlisle, seeking to end a mini-slump of four games without a win, were the better side in the first half and went ahead in the 34th minute. A mistake by Brighton defender Andrew Whing allowed Chris Lumsdon to release Simon Hackney, who scored with a rising left-foot drive.
Brighton were lack-lustre in the opening 45 minutes, but Forster hauled them level two minutes after the restart with a fine individual effort. Fed from just outside the penalty area, he evaded one defender but looked like being blocked by another when nifty footwork created room for a fiercely-struck right-foot shot.
Carlisle capitalised on more slack Brighton defending to regain the lead six minutes later. Danny Carlton's cross was missed by Joe Anyinsah but not Garner, who tapped it home from close range. O'Callaghan's early departure soon after seemed to have guaranteed Carlisle a return to winning ways but Forster had other ideas.