Oxford followed up their opening-day draw at Sunderland with a 1-0 home win over Peterborough thanks to Cameron Brannagan's first-half strike.
It leaves a much-changed Posh team with two defeats from two games.
Brannagan scored in the 12th minute with a superb left-footed volley from 15 yards after Alex Woodyard had headed out James Henry's right-wing cross.
Despite playing against a strong wind at the start, the U's were threatening. In the fourth minute, former Charlton winger Tariq Fosu dribbled past two defenders and shot against a post. Ben Woodburn fired wide, then drilled a dangerous low centre across the box.
Darren Ferguson's men showed more urgency after the break with Marcus Maddison's effort tipped over the crossbar by goalkeeper Simon Eastwood.
The visitors had more of the ball, but Oxford almost added a second on the break in the 69th minute.
Henry chipped the keeper only to see his effort come back off the bar, and from the follow-up Woodburn missed a near open goal.