The Black Cats secured victory through goals either side of half-time from the young winger, with debutant Adam Johnson providing both assists.
And O'Neill hailed the 23-year-old's contribution to the victory, before insisting that he was oonly withdrawn with the next game in mind.
"Both of James’ goals were well-taken – he did brilliantly for the first from the whipped-in corner," O'Neill told the club's official website.
|11/1||James McClean is 11/1 with Bet365 to score the first goal in Swansea City v Sunderland
The 60-year-old went on to express his delight at securing passage to the next round and reserved special praise for his side's League Two opponents.
"I’m delighted, I thought we played some really lovely football, particularly in the first half," he added.
"We’re in the hat for the next round, and that’s important. We came through with flying colours, I think. There were one or two anxious moments where we were a little sloppy, but overall it was very pleasing.
"Morecambe played very well. They came here and tried not play – not only that, they were competitive throughout the night.
"I was really glad we got that second goal to take any anxious moments away. I’m just delighted we’ve won."
The draw for the third round of the League Cup takes place on Saturday.