Jordan Bowery floored Pompey as Oxford took victory in the League Two promotion tussle today.
Sub Bowery struck with 14 minutes left as former Blues boss Michael Appleton masterminded a 1-0 win for his side.
That left Paul Cook’s men seven points off the top three as they suffered a painful defeat in front of a crowd of 17,840.
It also meant new keeper Ryan Fulton’s penalty save from Liam Sercombe amounted to nothing on his Football League debut.
It was a tight but intriguing game short on chances, but the U’s probably just about deserved the three points.
Michael Doyle got caught by a stray arm in the face from John Lundstam after two minutes.
That was the first of a number of niggling tackles on the captain, as he appeared to be targeted by the visitors.
Danger man Kemar Roofe headed over from a corner after eight minutes before George Baldock fired wide from a corner.
Oxford were having the better of it but Pompey had the best chance of the half as fine play from Gary Roberts put in Marc McNulty, but the Scot’s shot was cleared off the line by Jake Wright.
Kyle Bennett’s free-kick was deflected over the top after 24 minutes.
Referee Neil Swarbrick was target of the home fans’ ire for a number of dubious decisions, and Cook was left livid after one fair tackle from Gareth Evans on Callum O’Dowda which was penalised.
Liam Sercombe fired an angled drive wide six minutes before the end of a first half in which Oxford would have been pleased with.
That was reflected in them dominating possession - an extreme rarity against Cook’s Pompey.
The game sprung to life five minutes after the restart as Fulton conceded a penalty as he dragged down Hylton.
But the loanee redeemed himself with a magnificent double stop to deny Liam Sercombe, diving to his right and keeping out the follow-up which looked a certain goal.
That was the catalyst for Pompey’s best spell of the game as the home crowd raised their voices.
Despite the pressure, there wasn’t a clear chance to show for the play as Roofe replied by fizzing a free-kick across goal before sub Jordan Bowery dragged a shot wide.
Bowery then made the breakthrough with 14 minutes left as he latched on to a raking ball from Wright and angled his finish past Fulton from just inside the box.
A shot from Bennett couldn’t be forced home by Adam McGurk with three minutes left.
McGurk then planted his header at Sam Slocombe from Ben Close’s shot with seconds remaining.
But Oxford looked largely comfortable as they saw the game out.