But they missed the chance to move back into the automatic promotion spots after Portsmouth lost a top-of-the-table clash at Luton Town.
Second-half goals for Barnsley from Mamadou Thiam and Kieffer Moore cancelled out efforts from Josh Ruffels and Jamie Mackie at the Kassam Stadium.
Head coach Daniel Stendel named one change to the side that had beaten Rochdale on Saturday with Moore replaced in the starting line-up by Australian international Kenny Dougall.
Cameron McGeehan was the first to be frustrated by Simon Eastwood after being expertly found by Alex Mowatt. Adam Davies was not to be outshone as he superbly tipped Gavin Whyte’s drive against the bar.
But there was nothing the Welsh international could do when Josh Ruffels headed home in the 24th minute.
The Reds were caught on the counter moments later with James Henry’s lob hitting the bar and Davies blocked the rebound.
McGeehan was denied by Eastwood once again as Stendel’s side searched for an equaliser before the break. Towering centre-back Liam Lindsay let a brilliant chance go begging by heading over Mowatt’s pinpoint corner.
Top scorer Moore was called upon at half-time to try to force something from the game.
The woodwork came to the Tykes’ rescue once again as Davies parried Rob Dickie’s header on to the bar.
A terrible mix-up between Lindsay and captain Davies let Mackie in for an easy tap-in to double the U’s advantage.
Davies pulled off two superb stops to keep the deficit to two.
Thiam pulled one back 20 minutes from time with an easy tap-in after Jordan Brown burst in to the hosts’ box.
And Brown penetrated the U’s backline again to allow Moore to fire in to the top corner and level proceedings.
Oxford United: Eastwood, Hanson, Dickie, Nelson, Ruffels, Brannagan, Mourinho, Whyte, Henry (Kashi, 85), Graham (Garrett, 81), Mackie. Unused substitutes: Sykes, Long, Stevens, Carruthers.
Barnsley: Davies, Cavare, Pinnock, Lindsay, Williams, Brown, McGeehan, Dougall (Moore, 45), Mowatt, Thiam (Bahre, 71), Woodrow (Green, 61). Unused substitutes: Greatorex, Pinillos, Jackson, Adeboyejo.
Referee: Kevin Johnson.