My husband had his radiation at St. Joseph's in Santa Ana. Dr. Ash was the RO (although I don't know that I'd recommend him specifically at this point...).
We also interviewed Kevin Lin (RO) at Hoag at the Irvine office.
There are definitely options for radiation oncology in OC, and from Costa Mesa you'll have several to choose from in that central
location.
Let's hope you don't need to consult one!