Robert Duffy got the game's only goal as Oxford ended their 11-game winless run at the expense of 10-man Gravesend.
Jim Smith's Oxford kept up the pressure on leaders Dagenham
The striker thundered a rising header home on 36 minutes from Daniel Rose's teasing cross for his 17th of the season to settle a hard-fought game.
Defeat was harsh on Gravesend, who went close through a long-range effort from Charlie MacDonald in the first half.
But the visitors' hopes of getting a draw were ended on the hour when Peter Hawkins saw red for a second booking.
Oxford Utd: Turley, Quinn, Phil Gilchrist, Phil Gilchrist, Corcoran, Johnson, Anaclet (Pettefer 78), Hargreaves, Martin Foster, Rose (Burgess 65), Duffy (Robinson 68), Odubade.
Subs Not Used: Tardif, Luke Foster.
Goals: Duffy 36.
Gravesend: Cronin, Ross Smith, McCarthy, Hawkins, Ledgister, Opinel, Long (Keeling 74), Quinn, DeBolla (Coleman 45), MacDonald, Moore (Sodje 86). Subs Not Used: Mott, Ricketts.
Sent Off: Hawkins (60).
Booked: Hawkins, Opinel, McCarthy.
Ref: C Sarginson (Staffordshire).