The 21-year-old was a vital part of Guardiola's final season at City in 2025/26, scoring nine goals, despite playing most of the campaign out of position at left-back.
The club showed their faith in the England international with a long-term contract extension last summer and he backed that up with goals in their EFL Cup final win over Arsenal.
"Pep was a great manager. He put his trust in me, which I can only be grateful for. He helped me so much on and off the pitch as a person. If it wasn't for him, I wouldn't be sat in this position right now. Thank you to him.
"It means a lot obviously to get recognised by the players that you play with and play around. It's a huge award to win. Obviously, the players that have won it before have gone on to do massive things themselves."
O'Reilly becomes the fourth City player to win the award in the Premier League era after Leroy Sane, Raheem Sterling and Phil Foden.