Match ends, Oxford United 0, Portsmouth 1.
Oxford remain without a point from their first four League Two matches following defeat at home to Portsmouth.
Craig Westcarr scored the only goal of the game when he bundled home a cross from the left by Nicky Shorey.
Danny Hylton had the best chance for the home side as he hit an effort against a post just before the break.
The result leaves Michael Appleton's side bottom of the table, while Portsmouth sit second with 10 points, just two behind leaders Morecambe.
Oxford's latest defeat means they have lost eight successive league games for the first time in their history.
Appleton was in charge of Portsmouth when they were relegated from the Championship in 2012 and left to take charge of Blackburn six months later.
Portsmouth manager Andy Awford:
"I think the good teams win when they don't play at their best. In the first half we didn't play at our best.
"The standard wasn't quite what we've set in recent games, but what I will say is they dug in.
"It's a really hard-earned victory. To come away from home and get three points is always a big thing."
- 14RuffelsSubstituted forPotterat 84'minutes
- 7RoseSubstituted forO'Dowdaat 73'minutes
- 9MorrisSubstituted forJakubiakat 84'minutes
- 11HolmesBooked at 22minsSubstituted forJ Wallaceat 45'minutes
- 20WestcarrSubstituted forWhatmoughat 86'minutes
- 14StoreySubstituted forTaylorat 64'minutes
- 8J Wallace
- Darren Bond
- Shots on Target
Second Half ends, Oxford United 0, Portsmouth 1.
Foul by Danny Hylton (Oxford United).
Paul Robinson (Portsmouth) wins a free kick in the attacking half.
Delay in match Jed Wallace (Portsmouth) because of an injury.
Attempt missed. Junior Brown (Oxford United) header from the right side of the six yard box is just a bit too high.
Substitution, Portsmouth. Jack Whatmough replaces Craig Westcarr.
Tom Newey (Oxford United) wins a free kick on the right wing.
Foul by Ryan Taylor (Portsmouth).
Substitution, Oxford United. Alex Jakubiak replaces Carlton Morris.
Substitution, Oxford United. Alfie Potter replaces Josh Ruffels.
Foul by Carlton Morris (Oxford United).
Nigel Atangana (Portsmouth) wins a free kick on the left wing.
Corner, Oxford United. Conceded by Paul Jones.
Attempt saved. Junior Brown (Oxford United) right footed shot from the left side of the box is saved in the bottom right corner.
Goal! Oxford United 0, Portsmouth 1. Craig Westcarr (Portsmouth) right footed shot from the centre of the box to the bottom left corner. Assisted by Nicky Shorey with a cross.
Danny Hylton (Oxford United) wins a free kick in the attacking half.
Foul by James Dunne (Portsmouth).
Substitution, Oxford United. Callum O'Dowda replaces Danny Rose.
(Oxford United) wins a free kick in the attacking half.