Gillingham clung onto all three points at Priestfield despite finishing the game with just nine men.
Lewis Montrose’s strike from the edge of the box gave the home side the lead just before the break, but they were down to ten men minutes after the restart when Joe Martin picked up his second yellow of the afternoon.
Montrose saw red for a late challenge in the 88th minute, but Gillingham held on for a morale boosting victory.
Oxford United can have only themselves to blame, as they missed a number of golden opportunities, and could not break down an unchanged Gillingham side, even when they had a man or two men advantage.
Alfie Potter was United’s chief culprit, the winger spurning several chances himself. Potter’s first opportunity came when his lively run down the left ended with a cut into the box, but his effort flashed across goal.
Simon Heslop fired wide moments later, while at the other end, United keeper Ryan Clarke parried a strike from Jo Kuffour inside the box. Potter had another half-chance cut out by Garry Richards.
Joe Martin was rightly booked for a reckless challenge on nineteen minutes and from the resulting free-kick, Potter was again thwarted by a scrambled clearance.
United were having a purple patch. Robbie Hall dispossessed Richards on the left, but his shot was beaten out by Gillingham keeper Ross Flitney.
Just after the half-hour, Potter hit a left-foot shot wide when well-placed, then set up Peter Leven with a dink into the box, but Leven was denied by Flitney.
With half time looming Gillingham took the lead, somewhat against the run of play.
Clarke did brilliantly to parry a Kuffour strike, but as the ball fell to Lewis Montrose, he took one touch before hammering the ball past the outstretched right hand of Clarke into the top corner from 25 yards.
With the second half just minutes old Gillingham were soon reduced to ten men as Martin’s alleged reckless challenge on Hall resulted in a second yellow card. This seemed to galvanise Oxford and they had a good period of possession without any real end product
Simon Heslop was denied by a splendid save from Flitney, while Hall jinked down the left hand side, setting up Davis to drill a shot across goal.
United should haved levelled on 68 minutes, sub James Constable worked his way into the box, pulled the ball back to Potter, who once again had his effort blocked, this time at point blank range by Gillingham skipper Matt Lawrence.
Even when Gillingham were reduced to nine men, Montrose receiving a straight red card for a horrendous foul on the half-way line, United could not break through.
Constable fluffed a chance, chipping over the bar when one-on-one with Flitney and Gillingham hung on for a well deserved three points.
FULL TIME: GILLINGHAM 1-0 OXFORD UNITED
Saturday’s victory sees Andy Hessenthaler’s side climb to fifth in League Two with a trip to Swindon Town next up on Tuesday night.
Gills line up: Ross Flitney, Matt Fish, Garry Richards, Matt Lawrence, Joe Martin, Chris Whelpdale (Charlie Lee 65), Lewis Montrose, Curtis Weston, Luke Rooney (Andy Frampton 74), Jo Kuffour, Frank Nouble (Danny Jackman 52).
Subs not used: Paulo Gazzaniga, Stefan Payne.
Goal: Montrose (45).
Booked: Martin (19).
Sent Off: Martin (49), Montrose (88).
Oxford line up: Ryan Clarke, Damian Batt, Paul McLaren (Asa Hall 78), Michael Duberry, Jake Wright, Simon Heslop (James Constable 55), Deane Smalley (Danny Philliskirk 68), Alfie Potter, Peter Leven, Liam Davis, Robert Hall.
Subs not used: Andrew Whing, Steven Kinninburgh.
Booked: McLaren (25), Philliskirk (71), Potter (79), Wright (82), Hall (90+4). Duberry (90+5).
Referee: Mr Gavin Ward
Assts: Mr Ryan Atkin, & Mr Elliott Kaye
4th Official: Mr Paul Kelly
Attendance: 5,819