Billy Kee's late strike followed up Damian Batt's own goal as Torquay secured a 2-0 victory despite playing with 10 men for 73 minutes.
Lloyd Macklin was sent off in the 17th minute for flicking a kick out at Anthony Tonkin and, although Oxford right-back Batt had headed into his own net from Kevin Nicholson's free-kick 12 minutes earlier, the Gulls were facing an uphill battle.
James Constable missed a golden opportunity to level when he went through on goal, from Sam Deering's threaded pass, but blazed over.
The visitors nearly doubled their advantage just before half-time, but two brilliant saves from Us stopper Ryan Clarke denied Kee.
But the Devon side kept finding huge gaps on the counter-attack, and substitute Kee sealed it a minute from time with a low shot from 15 yards.
It was a second straight defeat for Oxford, who were hammered 5-0 at Bradford at the weekend, and they are finding their honeymoon now well and truly over after winning promotion through the play-offs.