Pompey saw off Oxford to grab another precious win in the race for promotion from League One.
There were just 88 seconds on the clock when Callum Lang's strike was fumbled into the net by Jamie Cumming to give John Mousinho's men the lead.
But the visitors fought back and Cameron Brannagan levelled from the spot, while Will Norris made some key saves.
It was the hosts who emerged triumphant, however, as Christian Saydee came off the bench to slot home his first league goal for the club.