Louie Sibley came off the bench to fire his first Oxford goal and deny Portsmouth a first win of the season in a 1-1 draw at Fratton Park.
Mark O'Mahony had stooped low to head Pompey into a 58th-minute lead, after Elias Sorensen had seen an early penalty saved.
Sibley brilliantly finished from the edge of the area 128 seconds after coming on as Oxford drew on the road for the second time in a week, after starting the season with three away defeats.
Pompey remain winless on their return to the Sky Bet Championship, their worst start to a campaign since 2009-10 - the season they were relegated from the Premier League.
Pompey boss John Mousinho designated the match as a "must win" against their fellow League One promoted rivals after suffering a 6-1 mauling at Stoke on Wednesday.
He made four changes from that humbling and they immediately worked as the hosts were awarded a third-minute penalty after Will Vaulks handled a Christian Saydee cross.
Dane Sorensen, on his first start since August, aimed at the bottom-right corner but within easy grasp for Jamie Cumming to thwart him.
Oxford responded strongly but Mark Harris kicked into his own foot after a pull back and Ruben Rodrigues sliced off target.
Austrian 'keeper Nicolas Schmid, who started for the first time for Portsmouth, got down smartly to tip Idris El Mizouni's low effort wide after a mazy run.
Neither Schmid or Cumming were overly tested by a series of tame strikes straight at them from Harris, Regan Poole and Saydee either side of half-time. But Pompey found a killer touch to take the lead in the 58th minute.
O'Mahony - who had only managed nine touches in a timid first half - flung himself at former U's winger Jacob Murphy's low cross to head into the bottom corner.
The on-loan Brighton striker almost scored a third goal of the week when he latched on to a Saydee through ball, but only thumped into the side netting.
United manager Des Buckingham made a double change in the 69th minute to bring Sibley and Dane Scarlett on, and three minutes later they were level.
Full-back Peter Kioso squared to Sibley, who took a couple of touches before lashing into the bottom corner for his first U's goal since arriving from Derby.
Portsmouth rallied and had four golden opportunities within a minute.
Saydee's long-range effort was parried by Cumming to Sorensen, whose follow-up was blocked to O'Mahony. The Irishman crashed his rebound under Cumming, but found Sam Long blocking on the line.
From the resulting corner, Connor Ogilvie leapt to flick a header against the inside of the post.
Siriki Dembele then sliced wide when clean through at one end before Murphy's powerful cross deflected off Kioso to force Cumming into a tricky near-post save - but neither side could grab a winner.
The managers
Portsmouth head coach John Mousinho apologised to assistant referee Bhupinder Singh Gill for an alleged discriminatory comment made from the stands during Portsmouth's 1-1 draw with Oxford.
Singh Gill drew attention to alleged offensive remarks made towards him during the second half at Fratton Park to referee Lewis Smith, with the club later appealing for information to help identify those responsible.
Smith paused the match and reported the comments to the fourth official before a Tannoy announcement was made to warn spectators about bad behaviour.
"A discriminatory comment was made to the linesman," confirmed Smith.
"I think there was something said to the linesman on the far side," said Mousinho about the incident.
"I've apologised to the linesman after the game if anything was said and I know that it will be dealt with appropriately.
"We heard the Tannoy announcement and everyone here will condone it."
Oxford head coach Des Buckingham added: "It is not my place to say. There were comments made which the referee felt he needed to report but I'll leave it to the referees to sort."
Portsmouth later released a statement saying: "Portsmouth FC are appealing for information from supporters after a racist comment was heard at today's game.
"The match was stopped in the 77th minute after the comment was directed at one of the referee's assistants from either Block E or F in the North Lower."
