Is OpenBSD Secure? (2019)

In a word: "no".

Back when I worked at AusCERT, there was a really nasty remote exploit discovered in OpenBSD. It was responsibly passed on to the OpenBSD folks who fixed it in a timely manner. However, because the exploit was never officially made public, they refused to publicly admit that it even existed or was ever a problem. Their remote exploit counter remained suspiciously unchanged -- it ought to have incremented.

While I do believe that they are doing their best and are far beyond the norm, I have little doubt that other "non-issues" have appeared over time.