Is it possible for another type of tumor to grow while I'm having chemo and radiation for oligoastrocytoma?

Yes but unlikely. It is very unlikely you would develop another type of tumor, but possible. If there is a new tumor in the same area, it's more likely the same type of tumor (oligoastrocytoma). Sometimes radiation and chemo causes changes on MRI that look like tumor growth but are not.
Chemo&ca growth. Well unfortunately in the world of medicine and especially in cancer...Anything can happen. The radiation will only treat within the area it is aimed. Chemo is a multitude of drugs and different cancers are susceptible to only certain types of chemo drugs. So theoretically the chemo might not cover a different cancer.