Hi Samantha, Kimberly here from landscape-solutions-for-you. Thank you for asking your question on a flower bed around your oak tree. Yes, you can put a raised bed around the five foot area. My crew and I have done this often.
You are right when you said to use top soil instead of clay. A good quality top soil is a key to a successful bed around a tree. If possible in your area, the best top soil to use is a mixture of sand, black dirt, and saw dust. If this is not feasible in your area, look for a top soil that is lighter weight with no extra weight bearing additives.
I usually recommend four to six inches of top soil to put around the entire tree. Use either shade or partial shade perennial or annual flowers. Add some organic mushroom compost to your soil. This is a great organic fertilizer that will last up to one year. It doesn't smell real well, but your flowers will love it.
Finish the bed off with mulch, pine straw, or decorative stones. Make sure you get on a water schedule to get the roots established.
Another idea is to consider a few Hostas and smaller Azaleas, such as the Gumpo. Adding a few dwarf evergreen shrubs will also give you color year around. Mix the flowers in with a few dwarf evergreen shrubs.
You will also be covering your oak above ground roots so it will look much more appealing. Thanks for asking the question here at landscape solutions for you. Let us know how you made out and what you decided on. Feel free to ask a question anytime. Have a great afternoon and evening.
Kimberly