Unless things are different now, you will usually have night call in oncology. I finished fellowship in 2004. Months like medical onc didn't generate too many calls, but a month like transplant usually had a call or two for fevers, etc. As I recall, it was heme, not onc, that generated the highest volume of night calls. On onc, 95% of the time the calls can be managed from home. The poor cardiology and GI fellows were always having to come in at night. I can't remember a time I saw, as a med student or resident, a Heme-Onc fellow at night!
Good luck, and please don't let the call in fellowship be the deciding reason you go into Onc!
Mike