Oxford recorded their second away success in the space of four days as they sent Leyton Orient crashing to their first home defeat of the season.
An early goal from Andy Scott and a dubious penalty converted by skipper Andy Crosby put the visitors in control - but then they had to survive a late battering and Lee Thorpe's last-gasp goal was not quite enough for the home side.
The visitors could not have wished for a better start as they took the lead after just nine minutes when the ever-dangerous Scott shook off Orient skipper Dean Smith and fired home from close range.
Orient struggled to find their form but almost levelled in the 35th minute when Gary Fletcher saw his header saved by Oxford keeper Andy Woodman.
As Orient started to take a grip on the game Carl Hutchings should have levelled just before half time but miskicked when he was just six yards out and then Jamal Campbell-Rice shot just wide.
Oxford stretched their lead nine minutes into the second half with a fortunate penalty when Scott looked to take a dive in the box but referee Phil Prosser thought otherwise and pointed to the spot.
Despite Orient's protests Crosby fired the ball home to the right of keeper Glenn Morris.
Orient took control as they threw caution to the wind and played with just three defenders but all they had to show for their efforts was a late strike that looked to go in off Thorpe as he and defender Crosby stretched to get on the end of a Campbell-Rice cross.
After the game Oxford chief Ian Atkins said: "It was a great result and I think we are going to get better and better."
While a disappointed Orient boos Paul Brush said: "It wasn't a good performance and I'm far from happy at the moment. There are too many inconsistencies in our play."