Mother in law had blood work done. b12 came back at 900. them she was told to see an oncologist. why is this?

Not because of this: There must be something else going on. That is not a high number. Perhaps this was ordered because of anemia or something else. A lot of oncologists also specialize in blood disorders, like anemia, or a low blood count.

Hematology-oncology: Many of us, practice as both a hematologist as well as an oncologist- so we do treat people with many types of blood disorders besides cancers. I think your mother in law probably is reffered to see a hematology/onc specialist perhaps for other reason- such as anemia/ low blood counts or other abnormal blood result. Her vitamin B12 level of 900 is within normal range.