Ji is an instant hero on Wearside after his winner against Manchester City.
O'Neill said: "Ji is only a young lad. He made his (full) debut against Wolves, he came on at the same time as James McClean against Blackburn and he will learn the game as he goes on.
"Let's be honest, he is coming on to play (against Man City) but really we are under the cosh so it was just to try to get him into positions where he could prevent them from making rushes through the middle and then to be in a position to try to receive it if we could get it out.
"I was delighted with him to score and he has very good feet.
"I know he slipped a couple of times in decent positions - for which I could have killed him - but he has really decent feet and will be a good player for us."