If your threat model is that you assume your OS vendor is part of the attack then you can't use a proprietary OS. There's probably no free OS either that can g

If your threat model is that you assume your OS vendor is part of the attack then you can't use a proprietary OS. There's probably no free OS either that can give you the confidence level you'd want, because you probably would want to have reproducible builds, a trustworthy update process (preferably with some kind of transparency log), a wide array of exploit mitigations (aslr/pie/pic, grsec kernel, maybe CFI). There's currently no free OS offering all of those.