The Latics went in front after just eight minutes following a clumsy challenge from Adam Virgo on Chris Taylor, and veteran Andy Liddell made no mistake from the penalty spot for his seventh goal of the season.
But struggling Brighton were back on level terms after 20 minutes when Steve Thomson put a delightful ball over the top for Glenn Murray to chase, and the Seagulls' top scorer outran Sean Gregan to fire in a powerful shot from 10 yards to take his goal tally to 11.
Three minutes into the second half the visitors were reduced to 10 men when Virgo was shown a second yellow card for a needless foul on Chris O'Grady on the touchline.
But Brighton hit back to cause goal threat in the 56th minute when Andy Whing's header was cleared off the line by Danny Whitaker.
Dutch keeper Michael Kuipers, who had made several smart saves, again came to Brighton's rescue in the 74th minute when he beat out a shot from the lively Taylor, but it was a disappointing second half by Oldham who just could not find a way to break down a stubborn 10 men.