Sunderland boss O'Neill happy to give Johnson minutes in Cup win
The summer signing from Manchester City has been missing with a thigh injury but was introduced with six minutes to go as he continues his comeback.
O'Neill said: "Of course McClean was excellent and scored a great second goal that allowed me to get Adam Johnson on the pitch for a few moments.
"It was one of those where I wanted to try and give him just a feeling for things again.
"In an ideal world it would have been great if he had a little longer, another aspect was of course that there was a chance of the game going into extra-time and there was a chance of him getting on."