Set-piece specialist Lauren Cresswell netted a superb brace as Belles made it two FAWSL2 wins in two.
However, the hosts had to come from behind as they were made to rue a collection of missed chances when United struck following a mix-up in the Belles area as Sahara Osborne-Ricketts slotted home in the 42nd minute.
Gordon Staniforth’s side were back on level terms as United’s ‘keeper picked up a backpass - gifting an indirect free-kick.
Cresswell stepped up to blast into the top corner for 1-1 five minutes into stoppage time at the Keepmoat.
A slow start to the second half saw the game burst into life when Cresswell curled a stunning free-kick into the top right-hand corner from all of 25 yards.
The impressive Reanne Thomas later lofted home her first for the club with 20 minutes to spare as United crumbled during Saturday night’s encounter.
And Oxford’s fate was sealed when Balby youngster Ashleigh Mills lobbed in the fourth to clinch her first for the club.
Belles: Hobbs, Cunningham, Little, Sigsworth, Lipka, Smith, Bright, England, Thomas, Roberts, Cresswell. Subs: Williams, Mills, E.Johnson, M.Johnson, Govier.
Oxford: Lambourne, Mobbs, Chivers, Connor-Iommi, Lane, Osborne-Ricketts (C), Hines, Stanley, King, Low, Allison. Subs: Cox, Dinker, Boxall, Franklin-Fraiture, Rickett, Fellows.