Westcarr wins it late on
The midfielder then almost found the net at the other end, with his defensive header form Danny Rose’s corner having to be cleared off the line by James Dunne.
First he palmed Joe Riley’s fiercely-struck free-kick over the crossbar before denying Michael Collins at point-blank range from the resulting corner.
Only a couple of superb saves from Paul Jones prevented the hosts from breaking the deadlock in just the second minute.
Miles Storey replaced Ryan Taylor up front, while Jed Wallace made way for Danny Hollands in midfield.
Andy Awford made two changes from the side that beat Northampton during the week.
But it was the Blues on top after the interval and Westcarr converted Nicky Shorey’s cross to snatch all three points.
The hosts looked strong in the first half, with Danny Hylton striking the woodwork just before the break.
Pompey’s fine start to the season continued as Craig Westcarr’s late strike secured victory at Oxford.
Pompey began to settle into the game as the match progressed, although fine crosses from Westcarr and Shorey were not taken advantage of.
Autumn appeared to descend on the Kassam Stadium midway through the first half as bright blue skies made way for strong winds and heavy rain.
Storey then used his electric pace to burst into the box, but United keeper George Long raced off his line to save the angled drive.
Joe Devera was beaten in the air and Jones had to react quickly to deny Hylton before the same player cracked the rebound against the post.
But it was Oxford who almost took the lead just before the break when a Rose corner caused havoc in the Blues box.
Oxford 0 Pompey 0 Half Time:
Then, in stoppage-time, Hylton went down in the box under Westcarr’s challenge, but referee Darren Bond waved away his furious appeals.
It came as no surprise when Ricky Holmes – who had been booked for one first half challenge and perhaps fortunate to be sent off for another – made way for Wallace before the restart.
The Blues were looking particularly threatening on the break and one quick move ended with Wallace’s low cross having to be diverted behind at the near post.
It was a very open start to the second half, as both sides looked to get forward whenever possible.
Pompey soon created a decent opening as Hollands’ chipped pass cleared the defence, but Long came racing from his box to beat Storey to the ball.
Awford made his second substitution on 64 minutes as Taylor replaced Storey in Pompey’s front line.
It was the first player he brought on that was posing the hosts plenty of problems, however, with Wallace constantly looking to run at the defence.
Hollands could not quite reach Shorey’s dangerous cross from the left, but WESTCARR was there to bundle over the line.
The former England international did play a key role in Pompey’s winner on 80 minutes, though.
But Shorey, on the opposite flank, almost became an unlikely goal hero when he cut inside and curled in a shot that Johnny Mullins was did well to block.
Jones then made sure the Blues kept their lead, getting down low to his left to palm clear Junior Brown’s goalbound effort.
And it was Brown who missed a golden chance to rescue a point in the closing stages when he headed over from just a few yards out.
Jones; Devera, Robinson, Chorley (c); Holmes (Wallace 46), Atangana, Dunne, Hollands, Shorey; Westcarr (Whatmough 87), Storey (Taylor 64) Pompey (3-5-2):
Crocombe, Meades, Hunt, Raynes Subs not used:
Long; Riley, Mullins, Wright (c), Newey; Collins, Ruffels (Potter 84); Rose (O’Dowda 73), Hylton, Brown; Morris (Jakubiak 84) Oxford (4-2-3-1):
Westcarr 80 Goals:) 6,852 (2,136 Pompey fansAttendance:
Darren Bond Referee:
Brezovan, Butler, Awford, Barcham Subs not used:
Holmes Booked: