Dean Bowditch's penalty had given Karl Robinson's men a slender half-time lead, but Powell's double and further strikes from Mathias Doumbe and Stephen Gleeson emphatically sealed their first win in six games.
Bowditch put the Dons ahead after nine minutes, sending goalkeeper Alex Cisak the wrong way from the penalty spot after Powell had been dragged back in the box by Jean Yves M'Voto.
Oldham rallied and Kieran Lee had a fierce 18-yard drive pushed wide by David Martin before fellow Latics full-back Paul Black left the field on a stretcher on the stroke of half-time after twisting awkwardly.
The Dons upped the tempo after the break and Powell doubled their lead just two minutes after the restart, racing clear onto Bowditch's chipped pass to coolly round Cisek and slot the ball home. Powell then completed his brace with a quick turn and shot after 66 minutes before Doumbe curled an exquisite left-foot shot inside the far post from a tight angle to make it 4-0 three minutes later.
Gleeson sealed the rout 12 minutes from the end when he controlled James Tavernier's deep cross on his chest and drilled the ball home. Substitute Jay O'Shea almost made it 6-0 when he hit the crossbar at the death.
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