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 ~