The 20-year-old Albion Academy product came off the bench to make his professional debut in the dying moments of Saturday's crucial 2-1 win at Swansea.
"It was amazing," said O'Neil to the WBA website. "I don't know whether to cry or laugh, it was unbelievable.
"I only got on for a short while but it is every young players dream. It was nerve-racking but it's something I've always wanted to do. Pepe has given me the chance.
"Hopefully I've taken it and shown him what I can do.
"It was incredible at the end and it's difficult to explain how it felt.
"I'd like to thank all the staff at West Brom and all the people who have helped me."
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