An oncologist is a doctor that treats cancer. Oncology is the study of tumors.
A radiation oncologist treats cancer with radiation. A surgical oncologist removes cancerous tumors. A medical oncologist treats cancer with medicines like chemotherapy.
Most urologists do a lot of things other than cancer treatment, thus would not be an oncologist. An urologist that only treats urologic cancers could be termed an oncologist.
Generally, we look to a medical oncologist to be in middle without a bias toward either surgery or radiation. Of course, this probably isn't a valid assumption in all cases as I would expect that many oncologists do have certain primary treatment biases. For PCa we generally enlist a medical oncologist when surgery or radiation either hasn't been or can't be curative.