Ronan Curtis breaks free of Oxford's Sam Long during the first half
The first match finished all square at Fratton Park on Friday night.
Pompey took the lead in the 37 minutes with a strike from Marcus Harness before Oxford United equalised during injury time at the end of the first half.
The teams were unable to be separated after 90 minutes and Extra Time.
Pompey take on Oxford in the second leg of the League One play-offs.
Pompey fell to defeat in the penalty shoot out losing 5-4.
See all the best photos from the first half action.
Pompey fans find a vantage point to watch at the Kassam Stadium Pompey's Ronan Curtis breaks free of Mark Sykes of Oxford United Ronan Curtis breaks free of Mark Sykes of Oxford United Mark Sykes of Oxford United and Pompey's Steve Seddon in action. Marcuss Harness skips past Alex Gorrin of Oxford United during the first half Portsmouth fans watching the game from a car park Marcuss Harness scores a goal to make it 1-0 (2-1 on aggregate) in the first half. Ryan Williams and Ellis Harrison rush to congratulate team-mate Marcuss Harness after he opened the scoring. Marcus Harness goes in for a tackle during the first half Ryan Williams is challenged by Josh Ruffels of Oxford United Ellis Harrison holds off Alex Gorrin of Oxford United. Sam Long of Oxford United clears from Ronan Curtis John Marquis is challenged by Rob Dickie of Oxford United Steve Seddon runs with the ball. John Marquis is challenged by Rob Dickie of Oxford United Christian Burgess holds off Matty Taylor of Oxford United. Cameron McGeehan is challenged by Matty Taylor of Oxford United