The now former Blues forward penned a two-year deal with Steve Cotterill’s side on Thursday, after failing to agree terms on a Fratton Park extension.
It’s a move which some Pompey supporters have taken in their stride, claiming the club had to move on if the 28-year-old was unwilling to accept their terms.
The majority of fans on our Facebook page, though, see it as another telling blow during a frustrating transfer window to date.
At present, Marlon Pack remains Pompey’s only new arrival this summer, while the club remain without any senior strikers.
Here’s a selection of the views shared on our social media platform...
Bob Wright: As normal, decent player not given a decent wage so leaves for one of our rivals. Bet he comes back to bite us.
I give up. Clearly Pompey are content to be stuck in the lower leagues spending decent money on the ground which is nice but not on players.
Terry Wallington: I give up Pompey are going no where this season.
Mark Hill: Cowley valued him at x, Shrewsbury value him at y.
So what, decent enough player but not good enough to be our best past d player. Thanks but no thanks.
Dan McClumpha: How can Pompey not afford wages for this man?? But Shrewsbury can?
C’mon Cowley! We need to start getting some signings through the door.
Spencer Gruchy Calvert: OMG embarrassing. The thing is he was a safe signing and we’ve let him leave. Absolute farce.
We better have some even better replacements or I can see another mid-table finish.
One more year of failure and fans will start to turn on the owners.
Cowley can only operate under a poor budget for so long until he walks.
Paul Dicken: He used Pompey last season as a shop window when Sunderland didn't want him.
Just a mercenary player. Good player but no real loss.
John Treleaven: Something is seriously wrong when players are choosing to go to the likes of Shrewsbury and Bristol Rovers than come to Pompey.
It needs sorting out now! No more lame excuses.
Steve Duke: What is going on at our great club?
We cannot sort personal terms out with a player like O’Brien but Shrewsbury can. No disrespects to Shrewsbury but how can that be?
It must be the Cowleys where the problem is. School teachers not professional managers.
I am really starting to worry for Pompey. This endless circle of 12-14 players out. New players in every season all on loans or short term contracts is not working.
Harry Wood: Disappointed to see O'Brien go, but hopefully we'll have a decent enough replacement lined up, time will tell.
David Leng: It speaks volumes when Shrewsbury can offer better terms. We have a billionaire owner.. I suppose that’s why he's a billionaire lol
Can see us mid-table again next season.
Steve Mac: Couldn’t even play back to back 90 min games for Pompey without his performance dropping
Nothing to see here.
Paul Carden: I wasn't expecting him to sign for Pompey but Shrewsbury signing him is a sad indictment of where we are.
Andy Lanc's: It wont change, last year was the same next year will be no change, our owners think we can move up with freebie and loans.
Simon Whittingham: How are we possibly losing out on players to teams like Shrewsbury???
Howard Ise: I guess a relegation battle is more exciting than mid table again.