There are separate Hematology and Oncology board exams. You can't sit for either of them until you pass the IM boards. There are no oral exams. Whether you need to sit for both exams will be determined by your training (some, like me only board in Med Onc and therefore cannot sit for Heme boards) or contract (most only require you to sit for one).
Everything else you want to know could have easily been figured out by reading this website which is the first link you get if you google "Hem Onc Boards."