A second half header from Elliott Moore was the difference between the sides after the first leg at the Kassam Stadium.
Peterborough United go into the second leg of the Sky Bet League One play-offs on Wednesday night with a slender deficit to make up after a Elliott Moore header from a set-piece earned Oxford United a narrow 1-0 victory at the Kassam Stadium on Saturday evening.
On May 4th, the force was with Oxford in the opening exchanges with Josh Murphy once again causing a few problems down the left, but only a Ruben Rodriguez volley from close-range, which was saved by Jed Steer was the result of that dominance.
Ephron Mason-Clark saw a shot saved by Cumming after good work from the lively Joel Randall on 17 minutes, before Tyler Goodrham whistled a fierce drive just past the upright at the other end as Posh only half-cleared a corner kick.
Moore headed over from another set-piece on 37 minutes but at the interval, it was goalless at the Kassam.
Posh made a change at the break with Kwame Poku replacing David Ajiboye and the former Colchester United man definitely provided an outlet down the right.
It was Poku who nearly made an instant impact as he received a pass from Jadel Katongo and turned sharply, but his effort went over the top.
On 53 minutes, the home side nudged ahead as a deep corner was headed back across goal by Brannagan into the path of MOORE who powered a header into the back of the net from close-range.
Harrison Burrows and Hector Kyprianou both saw deflected efforts claimed by Cumming before substitute Ricky-Jade Jones saw a flick at the near post kept out by the keeper after good work from Randall.
Ronnie Edwards headed over from a corner and Burrows saw a shot shovelled to safety by the keeper as Posh camped in the Oxford defensive third but aside from a cross from Poku which hit Jones, there was no more danger for the home side and they go to Weston Homes Stadium with a 1-0 advantage.
Oxford: Cumming, Brown, Moore, Brannagan, Harris, Bennett, Stevens, Goodrham (sub Bodin 76min) , Rodrigues (sub McEachran 70min), Murphy, Dale (sub Browne 70min) . Unused subs: Eastwood, Long, McGuane, Leigh.
Posh: Steer, Katongo, Burrows, Edwards, Knight, Collins, Kyprianou, Randall, Mason-Clark, Ajiboye (sub Poku 46min), Mothersille (sub Jones 59min). Unused subs: Bilokapic, Fernandez, De Havilland, Dornelly, Clarke-Harris.
Half-time: Oxford United 0-0 Peterborough United
Full-time: Oxford United 1-0 Peterborough United
Attendance: 11,125 (1,478 Posh fans)
Referee: Lewis Smith