That result moved the U's to 19th in the table, as Peterborough fell to a defeat at Wycombe Wanderers, despite taking an early lead.
Match preview
Oxford beat the Grecians 4-2 in their last match, thanks to goals from Cameron Brannagan, Sam Long and a brace from Kyle Joseph which pulled them two points clear of the relegation zone.
However, they followed that up with defeat at Sutton United in the EFL Trophy in midweek, having made a number of changes from the side victorious in Devon.
It was a tough start to the campaign for Karl Robinson's team, who were on a five-game run without a win before the Exeter triumph, losing three of their first four games this season.
The results and statistics all show that the games have been tight affairs, though, with Oxford yet to lose a league game by more than a single goal.
Fleetwood Town are the only team in the bottom half of the table to have conceded fewer than Oxford so far, but only the Cod Army, Cheltenham Town and bottom-of-the-table Morecambe have scored fewer.
They have not been clinical enough in front of goal, scoring just six times from open play in the league in their 12 games this season.
Two of their four wins have come at the Kassam Stadium, but they have not won there since the victory against Burton Albion at the beginning of September and lost to Wycombe there two weeks ago.
The Posh have had their own recent troubles with the Chairboys, failing to beat them on two occasions over the past week.
Saturday's defeat at Adams Park came as no real surprise despite Harrison Burrows's early strike as Peterborough have lost five of their last six games away from home.
That was followed up by a draw - and a subsequent penalty shootout defeat - in the EFL Trophy at the same venue in midweek, but the point was enough to see Grant McCann's side progress to the knockout phase of the competition.
Posh do find themselves in the playoff places, but have played at least one game more than the five teams immediately beneath them in the table.
They have conceded 14 goals away from home in the league this season, with only Burton Albion and Forest Green Rovers conceding more on their travels than Peterborough.
Captain Jonson Clarke-Harris has scored 10 times in his 14 appearances so far this season, which sees him leading the scoring charts, two clear of Exeter's Jevani Brown.
Oxford United League One form:
W
L
L
D
L
W
Oxford United form (all competitions):
L
D
L
L
W
L
Peterborough United League One form:
L
W
W
D
W
L
Peterborough United form (all competitions):
W
W
D
W
L
L
Team News
Oxford will be without the injured Sam Baldock and Josh Murphy this weekend, with manager Robinson confirming that Baldock will have a scan and Murphy is looking at one or two weeks on the sidelines.
They could welcome back Jodi Jones, as the winger has recovered from his groin problem that kept him out of the defeat at Sutton earlier in the week.
Peterborough should have midfielders Jack Taylor and Kwame Poku available for selection again after overcoming injuries that have seen them ruled out of both encounters with Wycombe this week.
Defender Kell Watts could get his first league start for the club after impressing in the EFL Trophy game, and Ronnie Edwards could be recalled after being named a substitute for the past four games.
Oxford United possible starting lineup:
Eastwood; Long, Moore, Findlay, Brown; Henry, Brannagan, McGuane; Browne, Taylor, Joseph
Peterborough United possible starting lineup:
Bergstrom; Knight, Edwards, Watts; Ward, Taylor, Fuchs, Butler; Burrows; Marriott, Clarke-Harris