An oncologist is a physician, not a nurse. Most nursing schools have hundreds of applicants and very few seats. I've heard of people with 3.9 GPAs getting rejected from nursing programs; medical school is even more selective.
If you do want to become an oncologist, you'll need to maintain a high GPA and have an awesome background of volunteer work and participation in clubs and such. It's not impossible, but it will be a lot of work. Follow your dreams!
~ No One Can Make You Feel Inferior Without Your Consent -Eleanor Roosevelt ~
What you do in high school will not matter for medical school. Your high school grades matter to get you into college and your college and MCAT grades matter to get you into Medical School. This, however, is a nursing site.