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.
Oxford's Matt Bloomfield:
"It feels like two points dropped because of the timing of the goal, of course it does.
"But we all have to remember we've put a point on our total today which is a positive against an extremely tough opposition.
"But because of the timing of the goal against it feels terribly disappointing that we haven't seen it over the line.
"We've waited a long time as a football club to get a penalty ourselves, and Cameron Brannagan put it away nicely, and it just felt there was a really good energy in the ground in the second half. The irony is that they then get one.
"I thought we were much more where we wanted to be in the second half, in the first half we weren't - we needed to be much more pro-active.
"We got the goal, had the momentum, had a couple cleared off the line. I think if we'd have got the second goal, the game would have been done and dusted in our favour."
Charlton's Nathan Jones:
"It was the poorest of decisions - sometimes they get swayed by the crowd, and emotions, and don't make clear decisions - and it was very disappointing for us. Saying that, we showed character, threatened and probably looked the most likely to win it in the end.
"Looking at the positives, it's seven points from three games and if someone had offered that to me for this week at 3pm last Saturday I'd have taken it.
"My players didn't listen at half-time, their players did, and we played into their hands. They sit off and want you to play slow-tempo football and then they break on you.
"We said to them, with the high line, turn them, get the ball in behind them, make them defend, put balls in the box and you'll get joy. We did that for the last 15 minutes and got joy. We didn't do it for the first 75 and that's why we were 1-0 behind, as well as the poor decision.
"It's another point for us towards our goal, and it's a massive point really because it keeps them at arm's length.
"But I'm very disappointed overall because we could have won that game, had we been at it."
