A stoppage-time penalty from Gary Twigg handed Oxford all three points after they dominated against Forest Green.
The home team wasted several chances, with Robert Duffy, Matt Day, Eddie Anaclet and Arthur Gnohere going close.
The game burst into life after the hour mark when Day struck the crossbar, with Lee Dodgson repeating the feat for the visitors just moments later.
But in injury time Twigg was brought down in the box by Ryan Robinson and he made no mistake from the spot.
Oxford Utd: Turley, Quinn, Day, Gnohere, Anaclet, Michael Standing (Pettefer 66), Hutchinson, Trainer, Jeannin, Duffy (Twigg 66), Ledgister (Odubade 46).
Subs Not Used: Tardif, Corcoran.
Goals: Twigg 90 pen.
Forest Green: Robinson, James (Afful 64), Tonkin, Jones, Preece, Lawless, Brough, Clist, Stonehouse (Dodgson 68), Beesley (Pitman 72), Fleetwood.
Subs Not Used: Burton, Giles.
Ref: G Sutton (Lincolnshire).