O'Reilly was played at left-back by England coach Thomas Tuchel for their 2-0 World Cup qualifying victory over Serbia.
He told BBC Sport: "I'm over the moon. Tonight was a very special night making my debut. Dream come true for me and my family.
"I want to thank everyone who has helped me get here.
"My mum, stepdad, sister, for them to watch me play here at Wembley is a crazy feeling."
On being told he would be starting, he continued: "Earlier on today. We had a meeting and the squad was announced, and I was in the team. I was calm and I was excited. Big, big moment. I was calm, level-headed and just ready for the game.
"Everything happens for a reason, and it comes at the right time. I am just happy!
"I take it game by game and day by day. Keep working hard and keep going."
On aiming for a spot in Tuchel's World Cup squad, O'Reilly concluded: "It is an aim of mine. I would love to be there. Just keep working hard and hopefully I'll be there."