And the Pompey boss believes his new striker arrival brings all the attributes to make his mark in the second tier, after tearing apart Premier League 2 defences.
O’Mahony made his Blues debut against Sunderland on Saturday, being introduced for the final 30 minutes after sealing a season-loan loan deal last Wednesday.
The 19-year-old was on the scoresheet for the Seagulls in his final outing for them last week, in their 4-0 Carabao Cup success over Crawley.
That follows on from an outstanding record at Premier League 2 level last term, where O’Mahony smashed in 10 goals in 11 outings.
Now the challenge for the Republic of Ireland age-group international is to show he has the credentials to succeed in the Championship, after he returns from under-21 duty for his country this week.
That is something Mousinho has a confidence O’Mahony can be successful doing in the months ahead.
Mousinho said: ‘Mark has ticked the box in terms of scoring goals in PL2, that’s brilliant – but the step up to the Championship is big – we think he can make it.
‘He’s done it through the youth levels and as an Ireland international. Then he’s done it for the first team with Brighton when he’s been given the chance, which is really pleasing.
‘He showed the ability and knack to score for the first team against Crawley in an atmosphere of a local-ish derby.
‘We were hopeful of getting him through the door, have managed to do that and are really hopeful he can boost our attacking options – particularly in Colby’s absence.
‘He’s a good size and has a physical presence, he has a lot of attributes we would want to see in a centre-forward. He holds the ball up, is physical and dominates in terms of challenges – but also has the knack of scoring goals and can link play really well.