I know technically all Linux distros are the same OS so please more granularity in your answer. Also, if there are certain OS's that are more secure, why aren't they more popular for THAT reason? I know nearly everything involves trade offs.
"The world's first operating-system kernel with an end-to-end proof of implementation correctness and security enforcement is available as open source."
Definitely expected this to be one of those joke responses where the linked thing takes no user input, does nothing, and exits quickly. It really would be the most secure, but the usefulness would be questionable, of course.
Why doesn't everyone use OpenBSD for certain things? The first job in tech industry I had was with an OpenBSD shop. The admin was quite security conscious to put it mildly. I'd hazard to say that his attitude and personality fit the OS, which is good. I respected the guy. I think you may be right that OpenBSD is the most secure. Is there a trade off? That is, maybe OpenBSD is a huge pain for some reason? I used this back in like the late '90s so don't really remember many details.
Is there a Linux distro that comes close to OpenBSD? What is it specifically about OpenBSD that makes it more secure and could something similar be done with a Linux distro? These only appear to be brain dead questions, they really aren't. These aren't the droids you're looking for.
"The world's first operating-system kernel with an end-to-end proof of implementation correctness and security enforcement is available as open source."
https://sel4.systems/