The best answer I have ever had to this question was:
"If you want to keep something secret, never EVER put it on a computer, ESPECIALLY a computer connected to a network (any kind of network). If you want to keep something secret, put it in writing on a piece of paper, and keep the paper inside a safe".
The older I get, the more I realize the wisdom of these words. Let's face it, a "generalist" operating system will always be exposed and vulnerable to something or other. This being said, there are "secure" operating systems but most of these are either experimental/academic or extremely expensive.
This is the appeal of OpenBSD: these guys offer, by default, a reasonably secure operating system, with many protections and hardenings built-in. And these are real-life hardenings vs real-life exploits and attacks. All the things that OpenBSD does, Linux can do as well, but none of the protections offered by OpenBSD in a standard installation are activated by default on most Linux distributions.
Install OpenBSD, get a hardened, reasonably secure system. Install Linux, get something that requires hours of work to secure. Is OpenBSD perfect? No. Is it more secure "out of the box" than Linux? Yes.