Three goals in the second half condemned Pompey to their first league defeat of the season yesterday at the Kassam Stadium.
Kenny Jackett named an unchanged team from the eleven who were narrowly beaten at Cardiff on Tuesday and they marginally had the better of the hosts in a poor first half which opened up towards the break.
Simon Eastwood's only real save of note in that period came when Kyle Bennett struck the ball from outside the box whilst both Brett Pitman and Jack Whatmough failed to score with headers.
Pitman, and the travelling Pompey supporters thought the deadlock had been broken when the former Bournemouth striker headed the ball past Eastwood but it was ruled out for a foul. From my viewpoint at the other end of the ground it seemed like a harsh decision but those who were close claim it was a clear push on the defender.
Oxford, for all of their possession, failed to test Luke McGee with Drew Talbot once again stepping up and making some crucial challenges.
Unfortunately the second half started in the worst possible fashion as an unmarked Christian Ribiero was given plenty of space to shoot at McGee from an angle and the Pompey keeper couldn't keep hold of the ball, allowing Wes Thomas (who else?) to poach the opeing goal.
Pompey struggled to muster any real comeback to that with Gareth Evans coming closest to an equaliser by striking the far post directly from a corner.
He was then lucky to escape a booking for simulation after going down far too easily in the box, and Conor Chaplin was also lucky to esape a similar punishment for the same action later on as the visitors became desperate.
The match was wrapped up when Gino van Kessel took the ball into the box far too easily before firing it into the bottom corner and some gloss was added to the scoreline when Josh Ruffles was played through on goal and slotted in a third in stoppage time.
The result has well and truly brought Pompey fans back down to earth and nobody is going to be under any illusion as to how difficult this division is going to be to get promoted from.
It's clear to see that Kenny Jackett needs to sign a midfield enforcer and he will know that. With Tareiq Holmes-Dennis likely to miss the entire season, another left back is also going to be required as Brandan Haunstrup had some good moments yesterday but his inexperience also showed at times, especially as he was well out of position for the opener.
However, as much as some people will claim that Pompey had various key players unavailable, yesterday's result wasn't simply down to a lack of personnel.
Pompey's play lacked a lot of flow once again and it seems like the outsiders are going to be proved right about how having to change playing style under a new manager is going to hamper any progress early in the season as it will take some time for Jackett to embed his ideas into his team.
Too many times the players gave the ball away cheaply due to a lack of communication and there were a few players you would have expected better from. Bennett and Danny Rose in particular were poor whilst Adam May seems to lack the strength to compete in the Football League at the moment. McGee won't want to see his blunder again, either.
The players can't shoulder all of the blame, however, as there were a couple of baffling tactical decisions from the gaffer.
Is anyone convinced that Kal Naismith can play as a centre forward? He was ineffective when asked to do a job there last season and had no impact yesterday, either.
Also, if Chaplin is to be brought on, it makes no sense to hoof the ball up to him when Curtis Nelson and Mike Williamson are going to win those battles every time. If anything, he should be playing behind Pitman and not the other way round.
If there was a positive to be taken from yesterday's game it was Talbot's performance. He is like a player reborn and he certainly has the fans on his side. It's no surprise that Pompey were opened up on the right when he was taken off because as much as Evans did a solid job at right back in League Two, he is going to find League One much harder.
Still, that was as good as it got as Oxford fans sang "we are top of the league" whilst Pompey fans left the Kassam Stadium yesterday but there's no reason for panic.
After all, the problems that Pompey displayed are able to be fixed whether it's a case of strengthening the squad or simply practising the new way of playing.
And Pompey have still started this season better than last, and look what happened at the end of that one.
Pompey Player Ratings
Luke McGee: 5 - A couple of good stops but costly error for the opener.
Drew Talbot: 7 - Gave everything and was more than solid at the back.
Christian Burgess: 6 - Made some vital challenges but also was caught out too often.
Jack Whatmough: 6 - Won most of his battles.
Brandan Haunstrup: 6 - Looked good in spots but inexperience showed.
Adam May: 5 - Unconvinced he can play at this level yet.
Danny Rose: 5 - Lost too many challenges on return to former club.
Kyle Bennett: 5 - Dallied too oten.
Brett Pitman: 6 - Offered a threat but will never chase the opposition around.
Gareth Evans: 5 - Not his best showing by a long way.
Kal Naismith: 5 - Wasn't as involved as a centre forward should be.
Carl Baker (Tablot 69): N/A - All this switch did was open up the game in Oxford's favour.
Conor Chaplin (Naismith 56): 5 - Had no service and therefore posed no threat.