Clubs voted in June to end the season early and settle the table on a points-per-game basis due to the coronavirus crisis, meaning that Pompey and United finished level on points.
After a slow start to 2019-20 that saw them win only one of their first seven games, Portsmouth slowly built momentum to climb the table into their eventual position of fifth.
It goes without saying that Pompey's form at Fratton Park was the key to finishing in the top six, with no side in the third tier collecting more points on home soil.
Prior to drawing 2-2 with Fleetwood Town in what proved to be their final match of the regular season, Kenny Jackett's men had won seven in a row on the south coast.
Indeed, going further back, Portsmouth are unbeaten in 20 home league matches since April 2019, winning 12 of those games.
Oxford have their work cut out if they are to come away from Fratton Park with a first-leg lead of any sort, then, though they have performed well on their travels this season.
The Us won away at Ipswich Town and Shrewsbury Town in back-to-back matches before the coronavirus pandemic brought a premature end to the season.
Those wins gave United a final tally of 60 points - or 1.71 points per game - which is the same number that semi-final opponents Portsmouth finished with.
Much like Pompey, Oxford started the season slowly and had just five points from the first 21 on offer, seeing them down in 20th place at one point.
Karl Robinson's men slowly climbed the table, though, and a run of five successive wins to see out the season moved them into automatic promotion contention.
Had they been given the chance to play their remaining 11 fixtures, Oxford may well have continued that momentum and finished in the top two.
As it is, however, they have to settle for a place in the playoffs and another meeting with Portsmouth, whom they have already played out a couple of draws with so far this season.
Portsmouth's League One form: WLWWLD
Portsmouth's form (all competitions): LWWLLD
Oxford United's League One form: LWWWWW
The four month break has given Portsmouth a chance to get their injured players back to fitness in time for the playoffs.
Oxford are without Nathan Holland, who sustained a long-term injury last time out and has since returned to parent club West Ham United.
Portsmouth possible starting lineup:
MacGillivray; Seddon, Raggett, Burgess, McCrorie; Naylor, McGeehan; Williams, Cannon, Curtis; Harrison
Oxford United possible starting lineup:
Eastwood; Ruffels, Mousinho, Dickie, Long; Rodriguez, Brannagan; Mackie, Henry, Fosu-Mensah; Taylor
We say: Portsmouth 1-0 Oxford United
Portsmouth have not lost at home in the league in over a year, while Oxford won five in a row to see out the regular campaign. Form will surely count for little after a four-month gap between matches, however, and we can see Pompey taking a slender lead into next Monday's second leg.