Charles Barkley better keep his head on a swivel -- Shaquille O'Neal says he'll follow Will Smith's lead and slap the hell outta his TNT co-host ... dropping the playful threat in a hilarious freestyle rap.
The Big Aristotle made the comment at his surprise belated birthday party at Steak Market in Atlanta on Wednesday ... when the crowd of about 250 people asked him to get up to the DJ booth and give them a set.
The NBA legend ended up grabbing the mic and spitting off the dome for about 10 minutes instead .