An oath generaly taken by doctors of medecine, concerning the general ethics of medecine. There are many aspects of the oath that aren't related to the topic of Body Integrity Identity Disorder. Some say that the oath itself is outdated and several variants have been suggested.
For more information about the oath itself, see: Wikipedia:Hippocratic Oath
 Hippocratic Oath and BIID
The main aspect of the hippocratic oath that is often mentionned in relation to BIID is the concept of "first do no harm". The laudable idea that a physician should not do anything that could harm their patient is fine, but can be also somewhat short-sighted when a society views health as pertaining only to the body.
With that perception, it is inevitable that doctors assert their oath when refusing to perform surgery such as elective amputations or spinal cord transections. These surgeries are indeed harmful to the body.
But if we consider health in a more holistic manner, encompassing both body and mind, it becomes much more difficult to assert the "first do no harm" aspect of the Oath when refusing surgery as an option. We suggest that in order to heal the spirit, harming the body is inescapable. We further suggest that the balance of harm/benefit is firmly tipped towards the benefit. This suggestion has been discussed by Johnston and Elliott in Healthy limb amputation: ethical and legal aspects and by Levy and Bayne in Amputee by choice: Body integrity Identity Disorder and the Ethics of Amputation.
If someone has a malignant cancer in the knee, surgeons would not hesitate to remove the limb. They are harming the body, for its greater good. BIID can be compared to a cancer, and removing the offending limb is for the greater good of the combination body/mind.