An oncologist is a medical professional who is an expert in diagnosing, treating, and preventing cancer. The treatment of cancer by an oncologist is dependent on the type of cancer, its location, size and how much it has spread to other parts of the body.

The different branches in oncology are:
Medical - treatment using chemotherapy or other medicines
Surgical - removal of the tumour by surgery
Radiation - treatment using radiation therapy
Gynecologic - treatment of cancers such as uterine, cervical, and ovarian cancer
Pediatric - treatment of cancer in children
Haematologisc - diagnosis and treatment of blood cancers

Clinical oncologists are specialised doctors who use radiotherapy and systemic therapy such as chemotherapy for the treatment and management of cancer patients. Medical oncologists are physicians who specialise in the management and treatment of cancer with the help of systemic therapies such as chemotherapy, immunotherapy, targeted drugs, or hormonal therapy.

To become an oncologist, you need to complete the MBBS degree (5.5 years). Then you can either do a three-year course of:
MS, followed by a three-year of MCh (surgical oncology) or
MD (medicine/ paediatrics), with a degree in DM (medical oncology), which takes three years or
MD in radiotherapy.
If you can't do MS or MD, you can get a degree in DNB (Diplomate in National Board).

A doctor who performs surgeries for cancer treatment is known as a surgical oncologist. They remove the tumour and nearby tissues by surgery to:
Diagnose cancer
Treat cancer by eliminating all or some part of it
Identify the extent of cancer
Determine if cancer has spread or is affecting any other parts of the body
Restore the appearance or function of the part