You have good reason to be concerned with this oncologist's recommendation. Unfortunately, he/she seems to fall into the majority of removing organs unnecessarily (some will even remove the uterus as well as ovaries and tubes for ovarian cysts). You may want to seek out a different oncologist, one affiliated with a completely different network of physicians as hospitals, if possible.
You should NOT need to lose ANY organs, not even the one ovary IF the cyst/tumor is benign. If you have a surgeon with good cystectomy skills, he/she can remove the cyst, send it for frozen section and wait to get the results while you are under anesthesia. If the result is benign, then you should be sewn back up and sent to recovery, all organs intact. My surgeon actually waited for the frozen section results after removing my whole ovary. Even though it came back benign (9.5cm mucinous cystadenoma), he proceeded to remove the rest of my "reproductive" system (which is actually for much more than reproductions...needed our whole lives). I have been suffering the many medically documented after effects ever since and these effects are progressive and associated with many increased health risks.
Regardless of who you choose, you can specify what can and cannot be removed and under what conditions. Unfortunately, the surgical consent forms for gynecology surgery are typically written to allow the surgeon to remove whatever he/she wants and far too often they are overzealous removing more than is warranted. The only way to truly protect your body/parts and your lifelong health is to revise the consent form to specify your wishes and have the surgeon sign off on your changes. Be sure to make a copy of the revised form. It is not uncommon either for them to present you with a form the morning of surgery sometimes even after sedation has been started. This can have disastrous results. So be sure to do all this before the day of surgery.
You did not mention if you are post-menopausal (PM) but even in PM women, most ovarian cysts and tumors are benign. And many go away on their own.
Please keep us posted.