Is there a difference between radiation oncology and radiation therapy?

No. A radiation oncologist is a physician trained to prescribe radiation therapy.
No other than the dz. Radiation or x-ray therapy is used mostly to treat cancer or malignancies which is referred to as oncology. Throughout the history of radiation therapy a number of benign or noncancerous diseases have been treated with radiation. Most use the terms interchangeably. Today we use radiation therapy to tx benign conditions such as keloids, heterotropic ossification, plantar fasciitis, &some B9 tumors.
Semantic. Subtleties...Oncology means study of tumor, and radiation oncology is the role of radiotherapy in the treatment of tumors. Therapy is 'treatment: focused" as opposed to modality focused. Since radiotherapy can be used in things other than cancer or tumor, some object to the oncology term. Others dislike therapy because it sounds so utterly unscientific.
Terminology. Radiation therapy is the delivery of radiation to treat an illness (usually malignant) radiation oncology is the study and science associated with the delivery of that therapy.