Former United defender Alan Knill put one over his old boss Ronnie Moore as he masterminded a welcome win for crisis-club Rotherham.
The Millers are still in the relegation zone, but now only trail Gillingham - who occupy the last 'safe' spot by a point.
Knill was a coach at Millmoor when Oldham mananger Moore became a folk hero before his sacking 14 months ago.
But Knill - thrust into the manager's seat after the dismissal of Mick Harford - was all smiles after sending Oldham to their first defeat in six outings.
Rotherham old boy Chris Swailes gave the home side the lead with a 20th minute own goal and Paul McLaren added the second five minutes from the end.