Pompey remain fourth in the League One standings after being held to a draw by Oxford.
Sean Raggett headed home Ronan Curtis' cross to open the scoring for Kenny Jackett's side, while centre-back partner Jack Whatmough then hit the woodwork.
But Whatmough was adjudged to have fouled Dan Agyei after the break and Alex Gorrin converted from the spot, as the contest finished 1-1.