The former Leeds United midfielder was handed a rare start and didn't disappoint in Villa's midfield.
O'Neill told Villa's website: "On the subject of the young players, I thought Delph's first half performance was excellent.
"I wouldn't have given him a chance of playing last night because he's been ill for the last two days.
"He declared himself fit and wanted to play. We thought we would see how long he could last. He wasn't too well at half-time but I thought his contribution was excellent.
"He was driving forward with an ability to pass it. He will only get better."