Weymouth ended their seven-match winless run by shocking Oxford, thanks to Gavin McCallum's goal.
Oxford keeper Billy Turley had to be alert early on to deny Wilfred Domoraud's shot, before Alex Jeannin flashed a shot wide at the other end.
Scott Doe rattled the crossbar with a header for Weymouth before McCallum put the visitors ahead, firing in from from James Coutts' corner.
Yemi Odubade blazed wide as Oxford went in vain search of an equaliser.
Oxford Utd: Turley, Day (Clarke 61), Foster, Quinn, Jeannin, Ledgister (Fisher 53), Murray, Trainer, Odubade, Duffy, McAllister (Pettefer 32).
Subs Not Used: Warrell, Corcoran.
Weymouth: Matthews, Doe, Critchell, Kitamirike, Gross (Challis 90), Barti, Browning, Robinson, Coutts, McCallum (Justyn Roberts 84), Domoraud (Beavon 79).
Subs Not Used: Phillips, Malcolm.
Booked: Critchell, McCallum, Kitamirike, Domoraud.
Goals: McCallum 39.
Ref: A Hendley (West Midlands).