Not that it did him any harm.
O'Nien had not long linked up with the Black Cats from Wycombe in the summer of 2018 when former boss Jack Ross asked him whether he'd ever played at right-back.
O'Nien had played midfield for Wycombe, but said yes to Ross's question - and then found himself having to learn on the job when the former Sunderland boss put him on on the right side of the defence.
He impressed and stayed there. And as O'Nien reflects on his remarkable rise, he believes one element of his time at Wycombe has helped him adapt to new challenges at Sunderland, whether that's playing at full-back for the first time or marking some of the Premier League's best strikers as a centre-half.
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"I think in my first year, I lied my way in at right back," he admits looking back.
"The manager asked, have I played there before? And I felt like the right answer was yes. And then little did I know that I'll be starting there the next game in the FA Cup! I think it was against Walsall at right back.
"So I did find myself in at right back and then I think I stayed in the team at right back for the next kind of year and a half, two years. Listen, I enjoy being on the pitch and I enjoy problem solving. And you have that in every single position.
"I think if you look into why I've kind of found comfort at centre back, it's probably because I learned my trade at Wycombe.
"My education at Wycombe was man to man and if you actually look at centre back you can kind of say that is a man to man job because it's kind of you versus a striker. So potentially, if you look into that deeply, that might be why I found comfort at centre back.
"You know, being a man to man at Wycombe it was set up and destroy and I think that's kind of the same principle as maybe at right back as well, because it's you versus a winger.
"And at centre back, it's the same principle. So I'm here to learn and if I find myself as a left winger in these next five years here, then I'll have a lot of learning to do!"
Even at 31, O'Nien says he's still learning every day. He leaves no stone unturned in his quest for constant improvement and believes he's benefited enormously from the training ground environment at Sunderland this season.
"Nine months in, I'm trying to still win the ball off Brian Brobbey, so it's like, I've got to find ways to problem solve and to get the ball off him," he said in conversation with BBC Radio Newcastle.
"Playing alongside Granit, if I'm in possession, he makes my life easier, if I'm out of possession, I've got to think, well, where am I positionally-wise?
"You know, I was out training today, quite a few of the boys are on international duty, and it's me and Reinildo, he's one of the best 1v1 defenders I've ever seen.
"There's intensity, the way he presses, I'm going out there today, 1v1s against him and Enzo, and I know I'm going to go home today, I'm going to watch my clips back, I'm going to see where I can get better and then I'm going to come in tomorrow and against these guys again.
"It's just a wonderful environment to be in. So competitive which is exactly what you want, and, yeah, we're in a good spot right now, we've got to keep kicking on."