Charlton slumped to their first defeat since losing to Millwall at the Den at Oldham on Tuesday evening.
An early strike from Ollie Banks was enough for the Latics to take all three points.
The Boundary Park pitch was in a poor state, and the Addicks were again denied a nailed on penalty, but they had enough chances to have won the game comfortably.
Karl Robinson freshened up his squad, bringing in Jake Forster-Caskey, Jordan Botaka, Stephy Mavididi and Josh Magennis for Jorge Teixeira, Ricky Holmes, Tony Watt and Lee Novak, who dropped to the bench.
Charlton had the opportunity to get off to the perfect start. An aerial pass from Forster-Caskey put Magennis clean through after 15 seconds.
With just Connor Ripley to beat, the Irishman fluffed his lines, and lobbed the ball over the bar.
Three minutes later, Ollie Banks smashed the ball beyond Declan Rudd from the edge of the box for the game’s only goal.
The Addicks were to have further clear chances, Botaka, Ezri Konsa twice, Forster-Caskey, Novak and Watt all went close, and Chris Solly was clearly pushed when well placed.
Oldham had their moments too, Ryan McLaughlin was foiled by Rudd when clean through, and Peter Clarke struck a post from a corner.
But for the third consecutive game, Robinson will feel frustrated. Nine points were there for the taking, but his team ended up with just two. Such profligacy is likely to cost them dear.
Charlton: Rudd; Solly, Bauer, Konsa, Chicksen; Crofts, Forster-Caskey (Watt 79), Botaka (Holmes 59), Aribo, Mavididi (Novak 82); Magennis.
Subs (not used): Phillips, Teixeira, Dasilva, Jackson.
Booked: Bauer 75 (foul).
Oldham: Ripley, Wilson, Gerrard, O’Neill, Banks (Fane 90), Taylor, Erwin (Ngoo 79), Hunt, Clarke (c), McLaughlin, Green.
Subs (not used): Law, Flynn, Croft, Osei, Obadeyi.
Goal: Banks 4.
Referee: Mark Brown (East Yorkshire).
Attn: 2,946.
TAGS: Charlton Athletic