The 26-year-old, who moved to Wearside from Wycombe in 2018, reportedly had interest from Championship clubs before deciding to extend his stay in the North East.
A key factor in his decision were those discussions with Johnson, as the Sunderland head coach laid out his plans for the player and the club.
Their exchanges undoubtedly involved O’Nien’s positional role, after the versatile midfielder was asked to play at centre-back for large parts of last season.
O’Nien may be an adaptable operator who is willing to do a job for the team, but clearly he feels his best position is in central midfield.
And that is the role he has been given this campaign, with the 26-year-old starting six out of seven league games in his preferred spot – while missing Sunderland’s 2-1 win at MK Dons through illness.
When the Black Cats were short of full-backs last month, it was Carl Winchester who lined up - and impressed - the vacant right-back role.
So has the decision to move O’Nien back into midfield been justified when he has also performed well in other positions?
Sunderland’s results certainly suggests it has, with the side winning five of their first seven league games and progressing to the fourth round of the Carabao Cup.
O’Nien’s performances have improved during the first month and a half of the season, and it now appears he is having more impact on games from the middle of the park.
In recent weeks, the former Wycombe man has formed an effective partnership with Dan Neil – when O’Nien has predominantly been the player dropping deeper to protect the back four in a 4-2-3-1 system.
According to Football database Wyscout, O’Nien has recorded the second highest number of interceptions per 90 minutes in League One this season , averaging eight per match.
As Neil put it, his midfielder partner is like a ‘busy bee’ who challenges for headers and does the dirty work, allowing his teenage team-mate to express himself further up the pitch.
O’Nien’s role changed slightly during Sunderland’s Carabao Cup win over Wigan as he was paired with Corry Evans in midfield.
While the duo were disciplined and sat alongside each other out of possession, O’Nien was given more licence to get forward when his side had the ball.
That was demonstrated when Sunderland’s box-to-box midfielder broke forward into the Wigan penalty area to make it 2-0, converting Niall Huggins’ low cross.
And while Black Cats playmaker Alex Pritchard picked his moments to drop deeper and exchange passes in the No 10 position, O’Nien was also more involved in the side’s build-up play.
The latter received 39 passes against Wigan, the highest figure he’s recorded this season, and finished the match with an 82 per cent passing accuracy.
Johnson is expected to revert to the promising O’Nien-Neil midfield partnership for this weekend’s game against Bolton, which will put more defensive emphasis on O’Nien’s role.
So far this season, a place in the engine room appears to be the best place for him.