If that were true you might also expect FreeBSD and NetBSD to have similarly low levels of exploits, and my general impression (not research, just reading around) is that they have had more exploits in that timeframe (privilege escalation bugs, whatever).

You can read more about OBSD's security by going to https://openbsd.org then clicking the "Security" link near the top left.

ps: FreeBSD seems to have more users than OpenBSD, and NetBSD seems to have fewer users than OpenBSD.