Andy Awford praised Pompey’s second-half display which yielded a fourth-successive victory.
Craig Westcarr’s 80th minute goal settled a tight against contest at Oxford.
In a game of few opportunities, Michael Appleton’s hosts did strike the post in the opening 45 minutes, but Pompey improved after the break.
And that was reflected in a controlled second-half display during which Westcarr netted the decisive goal to lift the Blues into second spot.
Awford said: ‘My half-time message was to keep the ball, we kept giving it away too cheaply.
‘In the second half we were a lot better and controlled the game.
‘Craig won’t mind how it went in, he worked really hard and we just felt getting Ryan on to help him with a bit of presence up top and it did.
‘Once we got in front we made a couple of tactical changes and defended well to keep it.’