Short answer: If your plan covers out-of-network medical care, parity requires it to cover out-of-network mental health and substance use care on comparable terms. It does not force a plan with no out-of-network medical benefits to add them.
Federal guidance is clear that group health plans and insurers need to include out-of-network providers and inpatient benefits to treat mental health or substance use disorders if similar benefits exist for medical or surgical care. The parity regulation sorts benefits into six classifications, including inpatient out-of-network and outpatient out-of-network, and the comparability test is applied classification by classification.
This does not force a plan that offers no out-of-network medical benefits at all, like many HMOs, to add them just for behavioral health. The rule is comparability: out-of-network behavioral health coverage must track whatever the plan does for medical and surgical care in the same setting, rather than being singled out for tighter limits.