The top two teams of the 2023–24 EFL League One, Portsmouth and Derby County, gained automatic promotion to the Championship.
Dion Charles put Bolton ahead in the 23rd minute, scoring from a Paris Maghoma cross.
Barnsley pulled one back when substitute Sam Cosgrove, on for Devante Cole, scored.
Josh Knight scored for Peterborough United on 41 minutes and a goal from the penalty spot from Cameron Brannagan drew the game level.
The tie finished 2–1 on aggregate in Oxford's favour taking them to Wembley to face Bolton Wanderers.