Hematologist/Oncolog. Both hematologists and oncologists treat lymphoma. For certain stages and subtypes radiation oncologists arre also helpful.
Oncologist. I placed my answer because a hematologist is an oncologist and vice-versa. They tend to know both cancer and blood problems.
Cancer specialist. Lymphoma is a cancer of the lymphnodes. Doctors who are trained in internal medicine, can go on to specialize in treatment of cancer. This is the subspecialty of hematology/oncology. Larger cancer centers such as sloan-kettering in new york or M.D. Anderson in houston will have oncologists who focus their practice on lymphoma, the most well known of which is hodgkin's disease.