Hayden Carter and Gain Bazunu have impressed the Blues attacker during their spells at Fratton Park.
The Blackburn Rovers and Manchester City loanees have been two of the stand-out performers for Danny Cowley’s side this season, with consistent high-quality displays.
The 20-year-old Bazunu has proven to be one of European football’s hottest properties, while defender Carter has enhanced his chances of playing Championship football next term.
And O’Brien, who has played with the talented pair since his deadline day arrival from Sunderland, has saluted their performances.
O’Brien told The News: ‘Hayden is so calm on the ball, he’s a cultured centre-back, right-back, and all-round defender.
‘He’s someone who likes to get on the ball, and play passes forward, get up the pitch and dribble and take shots.
‘He even almost put one in the top corner (against Bolton) in the last minute. He’s brilliant, just brilliant.’
During last week’s draw with Bolton, Carter was guilty of tripping Elias Kachunga inside the box – which saw Kieran Sadlier level the score from the spot.
Despite a slight blemish on the ex-Burton loanee’s outstanding record, O’Brien felt the penalty was dubious.
Although he was surprised that ‘the best goalkeeper’ he’s played alongside wasn’t able to save it.
O’Brien added: ‘I hadn’t watched the penalty back, personally. I don’t know if there was contact because he ran across him, or if the striker’s just been clever, I don’t know.
‘At the end of the day, the referee gave it because he’s obviously seen something there. I actually thought Gavin (Bazunu) was going to save it because I trust him 100 percent.
‘He’s the best goalkeeper I’ve played with. I actually thought there was a very good chance he’d save it. He didn’t on the day and that’s no problem.’