Charlie Kelman converted a 93rd-minute penalty for Charlton Athletic as Oxford United lost two precious points in the Championship relegation fight.
Oxford were just minutes away from recording their fourth straight win and pulling three points clear of the drop zone when Ciaron Brown pulled back Lloyd Jones to give away the spot-kick.
The incident mirrored Oxford’s goal when Kayne Ramsay needlessly barged over Will Lankshear to concede a penalty, which was scored by Oxford’s Cameron Brannagan.
Oxford could find themselves back in the bottom three if results go against them later on Saturday.
Charlton’s attack was blunted by losing striker Lyndon Dykes to illness just before kick-off and he was replaced by Miles Leaburn.
The first half was uneventful at the Kassam Stadium with Jamie Donley, who scored Oxford’s winner at Blackburn in midweek, firing a free-kick over the bar in the only effort on goal from either side.
The game improved after the break and home defender Michal Helik lifted his right-footed effort over the outrushing Will Mannion and the goal.
Then came Ramsay’s moment of madness with no need to barge over Lankshear as Sam Long’s ball sailed through to Mannion.
Lankshear went down, the referee pointed to the spot and Brannagan slotted home the 57th-minute penalty into the bottom left corner for his fifth goal of the season.
Now the game opened up and Jones showed real coolness to clear off the line from Lankshear after Mannion had got a hand to slow down his shot.
Charlton were still offering little in attack before Brown threw them a lifeline in added time when he pulled back Jones inside the box.
Up stepped Kelman to fire home his sixth goal of the season and leave Oxford facing the prospect of another weekend in the relegation zone.