Published: 16:40, 02 January 2017
Gillingham had another player sent off as they fell further behind the League 1 play-off race.
Cheyenne Dunkley put Oxford ahead from a corner and that second half strike proved to be enough.
Gillingham had returning defender Deji Oshilaja dismissed – their third player to see red in the space of two games.
Seven-goal striker Cody McDonald was dropped to the bench as boss Justin Edinburgh shuffled his team and formation for the visit of Oxford.
Suspensions to Bradley Dack and Paul Konchesky forced him into breaking apart a settled side that had been improving, albeit with a lack of cutting edge in recent games.
Oshilaja was back starting in defence for the first time since the Brackley Town cup debacle but his return was marred by a red card. Midfielder Mark Byrne ended a three-month wait for another league start.
The Gills reverted to 3-5-2 for the match, with Jay Emmanuel-Thomas and Frank Nouble upfront.
Gillingham boasted wins from their previous three league outings at Priestfield and went into Monday’s game unbeaten in five League 1 home fixtures.
Clear-cut chances were in short supply for both teams in the opening half, with the visitors having the better of the early opportunities. Rob Hall fired over and Chris Maguire shot into the side netting after being picked out running in behind the defence.
Byrne almost got on the end of a Baily Cargill ball forward for the Gills while Emmanuel-Thomas and Jackson both had shots off the mark. Nouble did find the target but, under pressure from the defence, his shot was comfortably saved.
The second half took some time to get going. Hall had a shot blocked on the hour by Cargill and it wasn’t long before Gillingham’s substitutes arrived.
McDonald and Rory Donnelly were both introduced as the Gills reverted to four at the back but it was the visitors who broke the deadlock with 66 minutes gone.
A short corner routine, which Oxford had been using all match, produced the goal as Alex MacDonald’s flag kick was worked to Dunkley and he managed to hook a shot past Stuart Nelson.
Gillingham’s hopes of a comeback were hampered by the dismissal of Oshilaja. He picked up his first booking early in the second half and got his second for pulling down Ryan Taylor as he threatened to break clear on goal.
The Gills were forced to play the final 14 minutes with ten men.
A great diving save from Nelson denied Roberts from making it 2-0 and Maguire smashed a powerful shot into the side netting.
Gills sub Donnelly put a free-kick into the Rainham End but was closer with an overhead effort that went just over the bar with four minutes left.
The Gills were leaving gaps at the back in search of an equaliser and John Lundstram put a great chance over on 88 minutes after Roberts found him in space.
Gillingham: Nelson, Jackson, Cargill, Ehmer, Oshilaja, Wright (Oldaker 78mins), Byrne, Wagstaff (Donnelly 62mins), Hessenthaler, Nouble (McDonald 62mins), Emmanuel-Thomas. Subs not used: Hadler, Herd, Knott, Osadebe.
Oxford United: Eastwood, Lundstram, Nelson, Maguire, MacDonald, Edwards, Hall (Roberts 71mins), R Taylor, Ledson, Johnson, Dunkley, Subs not used: Buchel, Skarz, Ruffels, Hemmings, Rothwell, Raglan.
Attendance: 6,414 (1,238)