Charlie Kelman's added-time penalty earned Charlton a 1-1 draw at the Kassam Stadium to deny Oxford a fourth straight Championship win in their battle to beat the drop.
Substitute Kelman tucked away the spot-kick in the third minute of added time after goalkeeper Jamie Cumming fouled Lloyd Jones as he came to collect a free-kick.
Cameron Brannagan had earlier given Oxford the lead in the 57th minute, also from the spot - the first penalty Oxford have been awarded in nearly two years.
Kayne Ramsay brought down Will Lankshear, and United skipper Brannagan slotted the penalty right-footed just beyond Will Mannion's dive and into the bottom left corner.
There was one enforced change for Oxford with Myles Peart-Harris missing out after taking a knock in the midweek 1-0 win over Blackburn.
Charlton boss Nathan Jones continued with his squad rotation, making five changes to the side which had triumphed at second-placed Middlesbrough, resting matchwinner Conor Coady, Miles Leaburn and Kelman among others from the starting line-up.
Both teams were in form and full of confidence, and Charlton made the brightest start, Greg Docherty firing in a low angled shot that Cumming saved with ease.
The Addicks had a lot of the ball in the opening 20 minutes but skilful play by Yunus Konak helped create an opportunity for United, Lankshear turning the ball on to Aidomo Emakhu only for Lloyd Jones to intercept smartly.
The surprise win at Middlesbrough had taken Charlton to 48 points and virtual safety, and there was freedom in the way they played in the first half.
However, Stan Mills began to have an influence on the match for Oxford. When he was brought down just outside the box, Jamie Donley curled the free-kick over the bar.
From Mills' dangerous right-wing cross moments later, Amari'i Bell sliced a clearance just past the angle of his own post and bar.
Early in the second half Michal Helik flicked the ball over the bar after Lankshear nodded on Brannagan's long diagonal cross.
Conor Coventry headed wide from a long throw by Charlton, but the home side then began to attack with increasing frequency, and when Lankshear drew the foul from Ramsay in the box, a huge cheer erupted in the stadium.
Lankshear almost added a second goal four minutes later when he chipped the ball over keeper Mannion only to see Lloyd Jones get back to clear off the line.
Matt Bloomfield's men looked to be heading to another vital three points, but the Addicks had other ideas - and made Cumming pay for his late error.
The managers
Oxford's Matt Bloomfield:
To follow...
Charlton's Nathan Jones:
To follow...
