 The OOP limit may be higher for out of network providers in a PPO (out of network providers are typically not permitted at all in an HMO).And as with all HMOs—no matter whether a person is on Medicare or not—persons who enroll in a Medicare Advantage or other Part C HMO cannot use certain specialist physicians or out-of-network providers without prior authorization from the HMO, except in emergencies.In almost all Medicare Advantage plans—HMO or otherwise—the beneficiary must choose a primary care physician (PCP) to provide referrals and the beneficiary must confirm that the plan authorizes the visit to which the beneficiary was referred by the PCP.