Pompey stretched their drawing run to four games after Oxford fought from behind to claim a point at the Kassam Stadium.
Marlon Pack bent home a superb free-kick midway through the first half that deserved to win any game.
It was not enough, however, as Sam Long nodded in an equaliser that could yet prove crucial in the hosts’ battle to beat the drop.
John Mousinho made four changes from the side that were held to a draw at Shrewsbury at the weekend.
Tom Lowery had recovered from a knee injury to take his place in midfield, while there were starts for Michael Jacobs and Paddy Lane in the wide areas, as well as Di’Shon Bernard at the back.
Joe Pigott, Reeco Hackett, Owen Dale and skipper Clark Robertson were the men to make way, with the quartet dropping to the bench and Pack taking the armband.
Neither side was able to find any fluency in the opening stages, which quickly switched from one end to the other without any notable chances.
A Joe Rafferty throw did lead to a spot of pinball in the Oxford box that ended when Ryan Tunnicliffe prodded the ball into the arms of former Fratton keeper Simon Eastwood.
Gatlin O’Donkor then saw a shot deflected into the side netting following good work from Kyle Joseph on the left.
There was a worrying moment on 14 minutes when Lane and Stuart Findlay clashed on the side of the pitch and tumbled over an advertising hoarding.
The pair landed on the concrete with a clatter, but both were quickly back on their feet and able to continue.
United then had a chance to forge ahead, winning possession and bursting forward through Marcus McGuane.
He found O’Donkor in space in the box and the striker turned neatly, only to fire wastefully wide of the post.
But it was the Blues who broke the deadlock on 26 minutes after McGuane had been penalised for a foul on Connor Ogilvie.
PACK stepped up to try his luck and curled a stunning 25-yard shot past the grasp of Eastwood and into the net.
Oxford then won a free-kick of their own and Cameron Brannagan’s effort was kept out by Matt Macey before being cleared.
But the sides did go in level at the break, with a corner from Brannagan finding Long in space to glance home a header.
Half Time: Oxford 1 Pompey 1
The second half took a little bit of time to get going, with neither side able to find much joy in the final third.
But there were then chances at either end in quick succession, as Colby Bishop met a Lane corner and saw his close-range header saved by Eastwood.
Oxford soon launched an attack of their own and when Macey failed to hold Joseph’s angled attempt, Marcus Browne was unable to convert the loose ball.
The home side went close again just past the hour mark, as Brannagan played a free-kick to Tyler Goodrham on the edge of the box and his effort was well saved by Macey at the keeper’s near post.
Mousinho then freshened up his forward line on 62 minutes, as Lane was brought off for Dale wide on the right.
But United were still looking the more likely to score and when Long swung in a cross, it was headed wide by O’Donkor.
A couple more changes followed as the visitors tried to wrestle back control, with Hackett and Louis Thompson on for Jacobs and Tunnicliffe.
They did have an opportunity when Ogilvie’s cross was turned back into the centre by Lowery and Dale’s shot was blocked by Findlay.
Oxford were desperate for another goal to put more distance between themselves and the bottom four, with substitute Billy Bodin firing over from the edge of the box.
Pompey threw on Pigott in their own hunt for a late winner, but ultimately the sides had to settle for a point each.
Oxford (4-2-3-1): Eastwood; Long, Moore (c), Findlay, Brown; McGuane, Brannagan; Goodrham (Anderson 83), Browne, Joseph (Wildschut 76); O’Donkor (Bodin 83)
Goals: Long 37
Booked: Brannagan
Subs not used: McGinty, Fleming, Smyth, Smith
Pompey (4-3-3): Macey; Rafferty, Raggett, Bernard, Ogilvie; Lowery (Pigott 90), Pack (c), Tunnicliffe (Thompson 70); Lane (Dale 62), Bishop, Jacobs (Hackett 70)
Goals: Pack 26
Booked: Lane, Bernard, Lowery, Raggett, Rafferty, Thompson
Subs not used: Oluwayemi, Robertson, Scarlett
Referee: Declan Bourne
Attendance: 8,345 (1,460 Pompey fans)