The tourists took England's four remaining second-innings wickets either side of a three-hour rain delay to win by an innings and 46 runs.
Peter Siddle made the breakthrough when he trapped Mark Wood lbw for six before Jos Buttler chipped Mitchell Marsh to mid-off for 42.
When play resumed in mid-afternoon sunshine Siddle bowled Stuart Broad for 11 and had Moeen Ali caught behind for 35 to finish with 4-35 as England were bowled out for 286.
The defeat had no bearing on the destination of the Ashes, with England captain Alastair Cook lifting the iconic urn amid fireworks, ticker tape and champagne after his team won the series 3-2.
The hosts, underdogs coming into the series, avenged the 5-0 whitewash down under in 2010-11 and have now won all four home Ashes series since losing in 2001.
Australia will take solace from the emphatic manner in which they finished the series, ensuring they sent captain Michael Clarke into retirement on a winning note.
Clarke was given a standing ovation by The Oval crowd as he led out his team-mates on the fourth day, with England resuming on 203-6.
He announced his retirement after the Ashes were surrendered in the fourth Test at Trent Bridge and will hand over the captaincy to batsman Steve Smith.
When Siddle took the final wicket, Clarke, who captained Australia in 47 of his 115 Tests, was engulfed by his team-mates before he and fellow retiring team-mate Chris Rogers held souvenir stumps aloft and soaked up the crowd's applause.
Clarke then darted across the outfield to acknowledge the travelling Australia fans, signing autographs and posing for pictures.