The Black Cats currently sit third in League One – two points behind Barnsley with a game in hand – while they also have a Wembley appearance against Pompey to prepare for on March 31.
O’Nien believes the Wearsiders’ squad has enabled them to compete on both fronts.
The likes of Aiden McGeady has 92 Republic of Ireland caps to his name, has three Scottish Premiership titles on his CV and Champions League experience.
Meanwhile, January arrival Will Grigg was part of the Northern Ireland squad that competed at the 2016 European Championships in France.
In total, manager Jack Ross had six internationals in his line-up when Sunderland beat Walsall last weekend.
That’s significantly more than what the Blues can boast, with Ronan Curtis their only current player with international experience to their name.
O’Nien claimed that experience and big-match know-how and could have a major bearing on Sunderland’s promotion ambitions and their Checkatrade Trophy match-up against the Blues.
The midfielder told Chronicle Live: ‘Having that experience has put us in a good position in the league, and having that to our advantage – I think a few of the boys have played there (at Wembley) – that can only benefit us.
‘We’ve got a good bunch of boys who have helped us get this far in the cup and the league, so if we can just keep that going the whole squad’s getting more experience collectively.
‘We’re in a good position at the moment so the atmosphere’s good.
‘We’re sitting well in the table, we’re in a final – everything’s good.’
Pompey can leapfrog Sunderland in the League One standings when they travel to Shrewsbury on Saturday.
Sunderland have no game because of international call-ups and currently sit two points above the Blues in the table.