And with Sir Alex Ferguson watching from the stands, Saints issued a clear warning that they could be a stern test for FA Cup opponents Manchester United.
Southampton made the breakthrough after just nine minutes as Adam Lallana hit a deep cross to Rickie Lambert and he calmly set up Alex Chamberlain for a close-range finish.
Dean Furman volleyed an Oldham chance off target, but Southampton were gifted their second goal after 38 minutes when Oldham goalkeeper Ben Amos allowed a 20-yard shot from Lallana to squirm through his dive.
The excellent Chamberlain almost added a third, although Oldham missed two clear openings when Oumare Tounkara headed against a post and somehow prodded wide from three yards. The visitors wrapped up victory in the 52nd minute when Guly Do Prado slammed in Lambert's right-wing cross.
Midfielder Richard Chaplow added the fourth with an emphatic drive from 16 yards and Lambert slotted in to pile on the misery for a totally outclassed Oldham side. Substitute Lee Barnard completed the rout four minutes from time as he rounded Amos and tapped into an unguarded net.