The Collaborative International Dictionary of English v.0.48:

oncotomy \on*cot"o*my\ ([o^][ng]"k[o^]t*[-o]*m[y^]), n. [Gr.
   'o`gkos bulk, mass + te`mnein to cut: cf. F. oncotomie.]
   (Surg.)
   The opening of an abscess, or the removal of a tumor, with a
   cutting instrument. [Written also onkotomy.] --Dunglison.
   [1913 Webster]