Do abnormal CBC results and a referral to an oncologist mean cancer is a big worry?

Not necessarily. Referral to an oncologist does not mean automatically it is a cancer. Majority of oncologists are also hematologists (doctor who deals with blood disorders). So, in your case, it is not necessarily to be a cancer, it could be just a common blood disorder. Good luck.
Not necessarily. Doctors often refer patients to an oncologist for evaluation and treatment of anemia, which is common. This would be found on a cbc. Other possibilities include abnormal white blood cells or low platelets. Blood cancers (leukemia) can be suspected based on a cbc, but hopefully your doctor would have shared that with you! It's poor communication to be referred without an explanation. Ask your doc!