O'Neal came off the bench and finished with five points and five rebounds against the Indiana Pacers, who won 96-94.
"It's good to be out there with the guys, but obviously there's a lot of rust," O'Neal said. "The rust will wear off and I'll get my rhythm back."
O'Neal, a key component to the Heat's NBA title run last season, had surgery on Nov. 19. The 7-foot-1, 325-pound (146-kilogram) center had played in only four games before he was injured, averaging 14 points and 7.5 rebounds.
The Heat have struggled without O'Neal, their record falling to 19-23, reports AP.
Despite the loss, Dwyane Wade was happy to have O'Neal back in the lineup.
"We're a much more powerful team with him," Wade said.