Not sure about a conflict of interest but if the medic is to also take the role of STC, there is no chance that he will have enough time to fulfill both of this role. The STC already has his work cut out trying to train, coach people and drive the HSE system on a rig. Medics are also very busy people as they do perform other duties apart from treating injured or ill people. Not possible in my experience unless you know something that i don't?

Both roles have many similarities, including (hopefully, if the Safety Advisor is a member of a professional body, which sadly many offshore sector employers don't see as important) ethical standards including the ability to deal with confidential information. I suspect this is the area where you think there may be conflicts of interest. If you check the IOSH Code of Conduct, that should reassure you about the standards to which an OSH professional is expected to work - though I fully understand (and deplore) that some offshore safety advisors have inadequate qualifications and/or are not members of any professional body.

The only "conflict of interest' in my experience has been regarding the classification of treatment under OSHA. If the medic is also responsible for safety at a site, then he is more likely to be tempted to under report in respect to the guidelines, regardless of the level of treatment administered. Well its saves management the effort, redefining OHSA reporting requirements each time.