The striker bagged his maiden Blues strike with his first home touch for Danny Cowley’s men, in Saturday’s 4-0 win over Doncaster.
The Republic of Ireland international hit the back of the net seconds after entering the fray in the 80th minute.
It’s a landmark few if any of his predecessors have equalled, in a home bow O’Brien will never forget.
The 28-year-old admitted his finish was one he actually knew little about, as he bundled home Ronan Curtis’ cross.
But, after singing a short-term deal until the end of the season, it showcased the qualities he’s out to bring to Pompey’s attack.
O’Brien said: ‘I didn’t actually know much about it.
‘I knew it was going to get crossed, I anticipated it to come across the goal and I touched it first.
‘I think the defender tried to block it and he’s taken me out, but a goal’s a goal.
‘I like to be off the back post and on the blind side of defenders.
‘It’s a case of arriving just on time and trying to keep out of the sights of defenders.
‘It’s about ghosting in behind them, that’s what I like to do.
‘That what I want to do and that’s how I like to slot them in.’
The goal was not the first memorable debut O’Brien’s had in his career, after scoring on his first Republic of Ireland appearance in Poland in 2018.
He added: ‘I scored for Ireland on my debut at international level - but it wasn’t my first touch. It wasn’t as good as that!’
After his deadline day arrival from Sunderland, the ambitions are clear for O’Brien between now and the end of the season - despite his new side being stuck in the middle of the League One table.
O’Brien said: ‘This is a massive club.
‘If we can get this club back to the Championship it will go above and beyond.
‘That’s the aim - and we have to put our all from now until the end of the season.
‘We haven’t got a massive squad, but we just need to give our all.
‘If we can get into the play-offs I can’t see anything stopping us.
‘With Fratton Park full I can’t see anyone beating us here, and I think we’ll get up.’
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