Oldham ended Leeds' unbeaten home league record thanks to three goals in 12 first-half minutes.
Reuben Hazell's half-volley gave Oldham a 28th-minute lead, and eight minutes later Neal Trotman headed in Neil Kilkenney's corner to double the score.
In the 40th minute, Lee Hughes helped Craig Davies' cross towards the far post and Andy Hughes clumsily directed the ball into his own net.
Leon Constantine finished a fine move on 46 minutes for mere consolation.
Leeds: Ankergren, Richardson, Marques, Heath, Hughes, Prutton, Thompson (Parker 56), Kishishev (Constantine 46), Carole (Flo 46), Beckford, Kandol.
Subs Not Used: Huntington, Westlake.
Goals: Constantine 46.
Oldham: Crossley, Stam, Trotman, Bertrand, Allott, McDonald, Hazell, Kilkenny, Taylor (Smalley 19), Hughes (Kalala 58), Davies.
Subs Not Used: Pogliacomi, Eardley, Thompson.
Goals: Hazell 28, Trotman 36, Hughes 41 og.
Ref: Mark Clattenburg (Tyne & Wear).