Comprehensive Documentation

Many bugs are caused by programers not understanding function behaviour, particularly error conditions (often failing entirely to check for them).
They need to be educated.

Comprehensive Documentation

Extensive programmer documentation in manual pages
Near-fanatical dedication to documenting function behaviour
Identifies dangerous functions
Provides examples of good use

The OpenBSD source code itself provides many examples of how to write securely.
e.g. Privilege revocation/separation