The Hippocratic Oath is a symbol of collective moral and ethical promises made by doctors. Though the verbiage has changed over the centuries, many modern students of medicine take some manner of oath to follow the guidelines established in the original document. For example, the ancient text made the physician promise not to perform abortions or euthanasia, while the modern oath has adapted for modern mindsets.
The modern oath is non-binding in practical matters but emphatic about the moral purpose of the medical profession. It focuses on treating the sick human rather than the disease, and participating responsibly as part of humanity. It's a solemn promise to provide care and healing, prevent disease where possible and treat individuals with respect and compassion.
Primarily, however, it is used as a barometer rather than strict regulation. Though commonly a part of a ceremony in medical school, it is not required to become a physician. It is a symbol of a general ethic as opposed to stringent rules.
If a doctor breaks any part of the oath it's a matter of conscience rather than law. The exception to this is how the doctor breaks the oath. If he is guilty of an actual crime, such as malpractice or neglect, he can and probably will be prosecuted.