Stan Mills scored a brilliant individual winning goal just a minute from time to give managerless Oxford a 2-1 victory at the Kassam Stadium.
Recalled midfielder Tyler Goodrham rifled the U’s in front in the 23rd minute with a thumping drive from 20 yards as Saints struggled to clear a corner.
Saints levelled six minutes later when centre-back Taylor Harwood-Bellis headed Leo Scienza’s free-kick from the right past Jamie Cumming.
It looked destined to finish as a draw, but Mills had other ideas, running with the ball 50 yards, then hitting a low angled shot past Gavin Bazunu.
The result means Eckert’s Saints have lost three successive away games, are 12 points adrift of the top two and five points from sixth.
“It’s disappointing because we wanted to start well into that block of four games. I think that we actually had a good start to the game,” he said.
“We controlled it well. Then we just, with the goal, lost the calmness that I think you need against teams like that. You need to be calm in possession.
“We continued in the second half, and it was back and forth and a very open game. It’s a very disappointing result. Very disappointing.
“Today, it’s a little bit more the nature of the game in the second half. I think we have the chance to control it a lot more with our possession.
“If we don’t do that and the game opens up, then it just becomes a running game. I think that’s not where our strength is necessarily.
“We just need to make sure that we play these games to our strength.
“We are now halfway through the season, and we are nowhere near the amount of points that we need if we want to play for something.
“The positive thing is that we play again in three days, and we need to make sure we start the second half of the season well.”