Greetings Melissa:
Some trees (e.g., oak, hickory, and K. coffeetree) are just now starting to show signs of bud swell and bud break -- just need a little patience as it has been a relatively cool spring here in the central and northern part of Illinois.
This is perfectly normal for some oak species. I'm confident your oak tree's tardiness will become a faint memory within the next 7-14 days.