It was a useful point for Richie Barker's men, who were thrashed 4-1 at Fratton Park by Oxford on the opening day of the season.
But Pompey were kicking themselves that they failed to return home with all three points, as Joe Devera missed an open goal 10 minutes from time when he failed to connect properly with a cross four yards out.
Both sides were restricted mainly to long-range efforts in a tight first half.
Patrick Agyemang had an early shot saved for Pompey, and Oxford's Aussie winger Ryan Williams tested goalkeeper Trevor Carson with a low angled drive.
Tom Newey went closest for the U's before the break when he headed a Danny Rose corner just over.
Oxford at least carried some threat once James Constable had come off the bench, Constable immediately creating an opening for Williams, who drilled a cross-shot across the face of the goal.
Pompey, cheered on by more than 2,300 travelling supporters, gave a debut in midfield to former England defender Nicky Shorey.
Williams drove a spectacular 18-yard volley just over in a rare moment of excitement.
By the end, neither keeper had been forced to make a difficult save.