England youth defender Ciaran Clarke was promoted to the bench for the UEFA Cup game against Zilina, whilst Delfouneso grabbed his chance with a debut goal.
"Some of these players are developing and I think it is always great," said O'Neill.
"Nathan Delfouneso has made his first team debut at only 17 years of age and that is great.
"There are one or two other lads who have performed brilliantly - Marc Albrighton in the Youth Cup for instance.
"He has had a few set-backs in terms of injuries but we are hoping that he will be back soon."