And Paul Cook is delighted with how the Blues’ number two is progressing.
The former Fratton Park apprentice initially arrived on trial in the summer, but quickly impressed to earn a month-by-month contract.
The 25-year-old has since made three first-team appearances, most recently the Checkatrade Trophy victory over Bristol Rovers.
That fixture represented a first clean sheet for O’Brien, who had relocated to Gosport even before his Pompey return.
Pompey are pleased with his progress and he will remain at Fratton Park until January at the very earliest.
It is a similar agreement to Amine Linganzi, having also originally signed a monthly deal.
Cook will assess the goalkeeping situation once again in January.
However, O’Brien has caught his eye, both on and off the pitch, since his arrival in August.
Cook said: ‘Liam is doing smashing, he is doing really well.
‘Our keepers work very, very hard with John Keeley, we are really pleased with all three of them.
‘Fordey has done really well although – I think he will the first to admit – we have all got a bit of improvement in us.
‘We will look at Liam again in January, but everything is going well, there are no issues there.
‘The kid is doing smashing and works very hard.’
O’Brien, who originally left Pompey in October 2010 under Steve Cotterill, has appeared in all three Checkatrade Trophy fixtures.
Against Reading under-23s, he saved three successive spot-kicks during a penalty shoot-out in which Pompey lost.