Compare the number of CVE vulnerability trends over time between Linux: https://www.cvedetails.com/vendor/33 and OpenBSD: https://www.cvedetails.com/vendor/97
It's not even close! It's nearly two orders of magnitude higher for Linux. This isn't anecdotal or “vague opinion” CVEs are facts.
You can ask the follow-up question: Why is that?
And there are many reasons. It could just be that Linux having more users/eyes means more bugs are surfaced ... But you need to dig deeper to understand why OpenBSD is so much more secure, the core team of OpenBSD proactively reviews the security of other OSes and when they learn something, they rapidly implement the feature/fix in OpenBSD.
Again, read: https://en.wikipedia.org/wiki/OpenBSD_security_features Many of the proactive security features OpenBSD has are not implemented by other OSes. And in the case of kernel-level Crypto, they won't ever be because US export restrictions.