Like Brewster said, oncology fellowship is not available out of Family med residency. There really aren't many subspecialties available out of FP, which is one of the reasons why many people do IM or Peds residencies instead. FP is a "true" primary care field, in that the only fellowships that are done after FP are related to primary care in some way -- sports medicine is one, and geriatrics is another available fellowship. BTW, sports medicine can also be done from peds and IM, and geriatrics can also be done from IM. I'm not sure about adolescent medicine -- I know you can do it from Peds, but I don't know about FP.
Long story short, if you want to do a subspecialty in the oncology area, you will need to either do IM or Peds first (or a Rad Onc residency, or general surgery for surg onc).