The Black Cats had trailed the Championship side by two goals following a disastrous first hour, but the introduction of Wickham spearheaded a Sunderland revival, which started when the former Ipswich man netted his first goal of the season from close range.
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He told reporters: "He [Wickham] was great, I was delighted. It was a great contribution and it was good to get the goal, that's a great confidence booster for him.
"I think that was the first time he's been on the field at the same time as Steven Fletcher so from that viewpoint it looks as if they could play together."
O'Neill was however disappointed with his side's first-half display, which saw them fall behind despite dominating for long periods.
He added: "I thought it was going to be difficult from the start, but we certainly contributed greatly to our potential downfall with the first goal, and I thought we were really sloppy in the first half.
"But a proper third round FA Cup tie finally came to the boil in the final 35 minutes of the game and, roared on by the crowd, we did brilliantly to fight back."