This paper is for anyone who must select an OS for a safety-critical medical system. It provides information to help with estimates of the real cost of choosing a Linux or QNX OS. It lists requirements identified by standards such as IEC 62304, ISO 14971 and IEC 61508, and offers comparative estimates of the effort required to meet these requirements. These estimates are for initial certification and pre-approval, subsequent re-certifications following OS upgrades, and ongoing costs.