As a hiring manager, I don't believe choice of OS is critical to join a team and make an impact. So I may not bring that up. If it is really critical to YOU as a candidate, then candidate needs to bring it up as soon as possible in a conversation.
In any case, this is all pretty academic. When I've been applying for positions, a tour of the office is always involved when interviews happen, and I can see for myself what OS the devs are using (as well as other working conditions).
That's how I knew my current position would require me to use Windows, and I was able to take that into account when deciding whether or not to take the job.
If working the job requires a specific OS you should mention it right upfront in the interview, or better yet the job description. Why is it the candidates job to telepathically guess at what hard requirements might exist? You have a requirement, state it.