Hello

To be a oncologist, you need to be a doctor after MBBS. I'm afraid it's not possible to be an oncologist after BSc Microbiology.

To refer more details, check this link :

https://www.careers360.com/courses/oncology-course

But, you can do research related to Cancer Biology after your MSc or you can go for integrated PhD in cancer related subjects. For more information, I'd suggest you ask a teacher at your own college for more opportunities in cancer biology. Research is always more interesting and eventful than being a doctor. Oncology is an interesting field to work in and I wish you good luck.