There are several private doctors in Oklahoma that accept cash payments for their work. One should ask practices such as Dr. James Cash in the Tulsa area, for example.

As far as I know, the majority of doctors and hospitals only take the co-payment in cash these days. It has been a long time since things were different: years ago, in the 1920s and 1930s, for example, people did pay cash for doctors' visits; it was very common in the era before health insurance was common in society. These days, however, most doctors have accepted the system of health insurers and health maintenance organizations. The one occasion when doctors do take money for services rendered is if a patient is uninsured-- some doctors expect uninsured patients to pay in cash. There are also a small number of doctors who find the current health care system too complex and have switched to requiring the patient to pay in cash. But these are the exceptions. For those who have insurance (whether from their job or self-pay), the doctors get their fees paid directly from the healthcare provider, while the patient handles any co-payment.

In the US, they don't. It's pharmacies that keep track of, and report, narcotics transactions. Other prescriptions, not so much. Your local rules may vary.

It is legal for a provider to have a cash price and a a different price for different insurance companies. So yes.