There's a lot of defense in depth approaches in OpenBSD, and as someone who's been on the mailing list for quite some time, I get to see them in real time.
https://isopenbsdsecu.re/mitigations/ does break them down fairly well, perhaps a bit pessimistically, but, if you take time to read through this, you can still see a lot of innovation coming from OpenBSD, and often before others.