O’Nien, who is naturally a right-back, has slotted into midfield and centre-back positions for the Black Cats in League One this season.
Sunderland boss Johnson is delighted with the positional fluidity the 26-year-old brings to the table and has played him at the heart of the defence and in midfield, though he is yet to deploy him in his natural position.
Johnson hailed O’Nien as the best utility player he has ever seen and stressed he has been outstanding in all the positions he has been asked to play in.
The Black Cats boss added that O’Nien epitomises all the quality he wants from his squad at the Wearside outfit, with his passion and intensity for the club on full display in everything he does on the pitch.
Asked what he thinks O’Nien’s best position is, Johnson told a press conference: “I think he could be the best utility player I’ve ever seen.
“Obviously, I have not seen him at right-back yet but at centre-half he has been outstanding.
“He was very good in attacking midfield role as well.
“And I think that what he does that impresses me so much is he is all in, every single attribute, he’s all in.
“Even when he falls down over, he falls down with passion an intensity.
“And that is saying credit to him and credit to his professionalism.
“He really epitomises what you want from a personality of your team.”
Sunderland are back on the road again in the league and are up against Crewe Alexandra at the weekend.
And Johnson’ side, who are fifth in the standings, will be determined to make it four league wins on the trot as they continue their charge for promotion.