Which kind of oncology are you referring to?
Keep in mind that there are many ways to be an oncologist.
- Medical oncologists are what most people think of when they think of oncologist. They often prescribe chemotherapy. This is a fellowship after IM. It's not terribly competitive, although certain hospitals obviously have more competitive fellowships (Sloan Kettering, MD Anderson, Harvard).
- Surgical oncologists perform surgery on tumors. This includes breast surgeons, who focus on (duh) breast cancer. Surgical oncology can be quite competitive. It is a fellowship after general surgery. You can also do a dedicated breast surgery fellowship instead.
- Radiation oncologists prescribe radiation treatment plans for cancer patients. They also focus almost exclusively on cancer. This is NOT a fellowship; this is its own dedicated residency. It is quite competitive and difficult to get into.
You can also specialize in cancer from urology (urological cancers - testicular, penile, scrotum, and kidney) or OB/gyn (Gynecologic Oncology - ovaries, uterus, vagina, vulvar). Gyn Onc is extremely competitive, as an FYI. Neurosurgeons and orthopedic surgeons often have dedicated cancer fellowship tracks as well.