The winners of Pompey's Bristol Street Motors Trophy tie with AFC Wimbledon will host Oxford in the last 16 of the competition.
John Mousinho's men welcome the Dons to Fratton Park on Tuesday, December 19 (7.45pm kick-off), with tickets on sale now.
And if they can see off the League Two side, then Oxford will head to the south coast for a fixture scheduled to take place in the week commencing January 8.
Mousinho's former club currently sit two places behind table-topping Pompey in the third tier standings.
They secured Bristol Street Motors Trophy progress with a narrow win at Forest Green on Tuesday evening.
| Bristol Street Motors Trophy Round of 16 – Southern Section |
| Peterborough United v Crawley Town |
| Brighton & Hove Albion U21 v Reading |
| Wycombe Wanderers v West Ham United U21 |
| Pompey or AFC Wimbledon v Oxford United |
| Bristol Street Motors Trophy Round of 16 – Northern Section |
| Derby County v Bradford City |
| Blackpool v Burton Albion |
| Accrington Stanley or Lincoln City v Bolton Wanderers |
| Doncaster Rovers v Wigan Athletic |