They created plenty of chances against an in-form Oxford outfit at a windy Fratton Park and were on top for most of the contest.
A confident penalty from Gareth Evans, awarded for a foul on Ben Close, looked to have secured all three points.
But United applied late pressure and salvaged a draw when Matty Taylor nodded home in the final minute of normal time.
Kenny Jackett stuck with the same side that were unlucky not secure all three points at Bristol Rovers the previous week.
The earlier rain had died down ahead of kick-off, but a strong wind was still swirling around the stadium.
It made life difficult for the veterans, Gurkhas and standard bearers, who paraded onto the pitch for the annual Remembrance commemorations, which included a bugler playing the Last Post.
And the players also had to deal with the tricky conditions, with the ball blowing out of play as Ryan Williams shaped to cross.
The Australian winger also bent a shot wide, while an Evans effort was deflected past the post as the hosts dominated the opening stages.
They almost broke the deadlock on 16 minutes, but brave blocks denied both Williams and John Marquis in quick succession.
Ross McCrorie’s delivery then took a deflection off Alex Gorrin, forcing former Fratton stopper Simon Eastwood into action to prevent an own-goal.
It was not all one-way traffic, though, and Christian Burgess had to deal with Josh Ruffels’ cut-back before a Tariqe Fosu corner was cleared by Sean Raggett at the back post.
But it was still the Blues looking more likely to score and Ruffels made a key block after Ronan Curtis had spun and fired in a shot.
Evans then managed to get a cross into the centre at the second attempt, but Marquis could only nod into Eastwood’s arms.
The visitors were forced to make a change before the break, as Ruffels limped off after McCrorie had been booked for a firm challenge on the left-back.
His team-mate Jamie Mackie had already received treatment for a cut head, angrily accusing Raggett of foul play.
Oxford then came close to scoring in stoppage-time – and it would have been a second unfortunate own-goal in the space of a week for Craig MacGillivray.
Mackie’s low drilled effort was cleared from the line by Raggett, but the ball ricocheted off the keeper’s back and trickled just past the post.
Oxford United 0
Pompey continued to look bright following the restart and McCrorie’s centre was behind both Marquis and Curtis in the centre.
The latter then looked to have taken Williams’ pass too wide, but still managed to hit a ferocious strike that cannoned against the crossbar.
But the hosts were presented with an excellent chance to finally take the lead just before the hour mark.
It was a lovely flowing move and when Marquis slipped a pass through to Close, the midfielder was clearly tripped by Elliott Moore.
Referee Ben Toner pointed straight to the spot and EVANS leathered home the penalty to leave Eastwood with no chance.
The lead might have been doubled moments later, as the Blues marauded quickly up the pitch through Williams and Marquis.
But the latter’s pass for Evans was not quite right and the midfielder’s shot was parried behind by Eastwood.
Still the chances kept coming and after Eastwood did well to parry Williams’ drilled effort, John Mousinho cleared behind before Curtis could convert the loose ball.
Oxford had a dangerous set-piece of their own on 73 minutes, but MacGillivray was able to parry clear Anthony Forde’s free-kick from the edge of the box.
They then won a corner that led to an almighty scramble in the Blues box and there were plenty of hearts in mouths before the ball was eventually got to safety.
A MacGillivray goal-kick might have led to an assist at the other end, but Williams could only lift his shot over the bar.
Oxford had been second best for most of the afternoon, only to exert plenty of pressure in the closing stages, winning a number of corners.
McCrorie – who was carrying a yellow card – was replaced at right-back by James Bolton for the final five minutes.
A free-kick from Forde then narrowly cleared the crossbar after the United substitute had tried to move the referee’s foam forward a few yards.
But Oxford did level on 90 minutes, as James Henry crossed towards the far post, where Taylor was waiting to guide home a header.
Ellis Harrison was immediately brought on for Evans to add another attacking option in stoppage-time.
Curtis crossed to the back post, but nobody could make a connection, while another delivery from the same player was nodded over by Williams.
But they were unable to find a winning goal, leaving most inside a sold out Fratton Park feeling deflated.
Pompey (4-2-3-1): MacGillivray; McCrorie (Bolton 85), Burgess, Raggett, Brown; Naylor (c), Close; Williams, Evans (Harrison 90+1), Curtis; Marquis
Goals: Evans 58 (pen)
Booked: McCrorie, Marquis
Subs not used: Bass, Downing, Haunstrup, Cannon, Harness
Oxford (4-1-2-1-2): Eastwood; Long, Dickie, Mousinho (c), Ruffels (Moore 42); Gorrin; Sykes, Henry; Fosu (Forde 70); Taylor, Mackie (Baptiste 70)
Goals: Taylor 90
Subs not used: Stevens, Agyei, Lofthouse, Lopes
Referee: Ben Toner
Attendance: 18,528 (1,747 away fans)