Oxford are the only side that Pompey have not yet faced in League One this season, but that changes at the Kassam Stadium this evening (7.45pm kick-off).
This fixture was originally supposed to take place in November, only to be postponed because of international call-ups.
The Blues were, as now, sitting top of the table back then, while their next opponents were just a point behind in second position.
Liam Manning had just departed to take charge at Bristol City, replaced by Des Buckingham, who had previously coached at the club earlier in his career.
United have struggled for consistency since that change, however, and are currently sitting in seventh spot.
They head into this fixture on the back of successive defeats to Barnsley and Bristol Rovers, leaving them eight points off the automatic promotion places.
It will be the second time that Blues boss John Mousinho has returned to his former club, with Marlon Pack netting in a 1-1 draw last April.
A close contest could be on the cards tonight, with all of the 11 most recent meetings between the sides either finishing all square or being won by a single goal.
TEAM NEWS
Callum Lang could make his debut after signing from Wigan, while Joe Rafferty has recovered from a neck injury and is available for selection.
But experienced skipper Marlon Pack is a doubt after illness kept him out of Saturday’s victory at Port Vale.
Tino Anjorin (hamstring), Josh Dockerill (knee), Regan Poole (knee), Alex Robertson (hamstring), Ben Stevenson (knee) and Zak Swanson (groin) are all definitely sidelined.
Joe Bennett (calf), Billy Bodin (Achilles), Marcus Browne (hamstring), Kyle Edwards (hamstring), Will Goodwin (quad), Sam Long (head) and Josh Murphy (hamstring) are expected to miss out for the hosts.
VIEW FROM THE DRESSING ROOM
Read the thoughts of John Mousinho ahead of the game and also see what Lang had to say about joining Pompey.
HEAD TO HEAD
Overall
Pompey: 27 wins
Oxford: 24 wins
23 draws
On the road
Pompey: 9 wins
Oxford: 19 wins
11 draws
TICKETS
Pompey have sold their full allocation for this game, so please do not travel to the ground if you do not have a ticket.
There is industrial action on the railways today and services are limited, so please check before you travel.
Click here to read a guide to visiting the Kassam Stadium that has been produced by the home club.
MATCH COVERAGE
Supporters at home and abroad can watch the contest live by purchasing an iFollow match pass and visiting the match centre.
Domestic iFollow subscribers can also enjoy audio commentary from Andrew Moon and Guy Whittingham as usual, but will need to purchase a match pass if they want to watch the game.
Click here for more information about using iFollow on a matchday and click here for information on downloading the app, so you can access content on your mobile device.
We will have key updates throughout the game on Twitter and a detailed blog via the match centre.
There will be post-match reaction on the website and our official YouTube channel, with highlights to follow on Wednesday.
Local listeners can hear commentary by tuning in to either Express FM or BBC Radio Solent (not available online).
LAST MEETING