Rovers tried to sign the 28-year-old striker from Sunderland in the dying embers of last summer’s transfer window only for an administrative error by the Black Cats to prevent the deal from going through.
So when O’Brien latched onto Ronan Curtis’s cross to make it 3-0 merely seconds after stepping foot onto the Fratton Park pitch it was another cruel twist of fate for a club now staring relegation to League Two full in the face.
O’Brien made his Pompey debut as a late substitute in last weekend’s 3-2 defeat at Oxford United after joining on a deal until the end of the season.
“I was just very happy to get on and play in front of the home fans – to score with my first touch was incredible,” said O’Brien.
“Hopefully that can be the kick-start I need to have a good end of the season and get a few more goals to help the team.
“I was in front of their defender and got the touch, but I couldn’t see what happened and was screaming for a penalty.
“Then I heard the crowd celebrating and realised I must have scored, so it was one of my weirder goals.”
He added: “Doncaster played well and had some chances, but the opener got us going, with that ball into the box from Marcus [Harness] and a great Reeco [Hackett] header.
“It was a very tough match considering they’re down at the bottom of the table and they made it difficult for us.
“But that’s two wins out of two, so it’s time to start kicking on, moving up the table and fighting for that play-off spot.
“We’ve seen other teams dropping points, but let’s just worry about ourselves and try to put a run together.”
The defeat leaves Rovers bottom of the League One table and nine points from safety.