O’Brien left the Stadium of Light to join up with Danny Cowley’s Portsmouth on deadline day, joining on a permanent deal.
The 29-year-old had just six months left on his deal with the Black Cats and had struggled for game time last campaign, failing to complete 90 minutes in the league since late October.
Having signed a short-term deal at Pompey, he was one of 12 players whose contracts expired at the end of the season on the south coast.
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The versatile attacker - who can play as a left-winger, striker, and right-winger - played at Montgomery Waters Meadow twice last season and scored Portsmouth's equaliser in Shropshire back in February.
Speaking earlier this year after his exit from Sunderland, O’Brien admitted that getting back playing regular football was one of the main reasons for his move down south.
He told the Portsmouth News in April: “No, I haven’t had any conversations with the club, yet. I’m not sure what’s going on there.
“Obviously I would like to stay, I’ve hit the ground running here and I love all the boys here.
“Everyone’s brilliant but there’s been no conversations yet, but hopefully something will pop up soon. We’ll see what happens."
O’Brien has also previously lifted the lid on his time at Sunderland under former boss Lee Johnson.
He The Portsmouth News: “It started well, and it just ended so badly. I just needed to get out of there because I was kind of rotting away. The manager here knows that, and he took me on, which I’m thankful for.”