O'Nien made his first appearance of the pre-season campaign on Saturday as Sunderland produced an emphatic 4-0 win at Harrogate Town.
The 26-year-old had missed the previous two games with a shoulder injury and to accomodate his return, Dan Neil dropped into an unfamiliar left-back role.
Johnson's gameplan this season is to operate with a six, an eight and a ten in central midfield and O'Nien is set to operate in the second of those roles.
However, Johnson said that O'Nien was fully aware that his versatility was one of the key reasons why the Black Cats invested in a new deal to match significant interest from Championship clubs.
As such, O'Nien's days in defensive roles are not entirely over and indeed he finished Saturday's friendly alongside Ollie Younger at centre-back.
"We definitely see him as a central midfielder - we have signed him as a number eight," Johnson said.
"But the reason Luke got an extra percentage increase on his wage, which enabled us to keep him, is because of his versatility.
"By having Luke who can play at centre-mid, centre-half, right-back, it means you can reduce your squad by one body, which means you don't have to pay relocation, an agent's fee, or anything.
"I'd rather have 20 quality first-team players than 25 average ones."
Sunderland want to recruit at least two new full backs before the campaign starts, and remain hopeful that Denver Hume will agree fresh terms.
Johnson said that he believes the club are 'getting there' in the search for new recruits.