OpenBSD's bootloader seeds the kernel, mixing in other entropy sources. High quality random numbers are available very early compared to other operating systems.

See Theo's talk from Hackfest 2014, an updated talk originally given at EuroBSDcon 2014:

https://www.openbsd.org/papers/hackfest2014-arc4random/index...

Page 19 and beyond explain this in detail: https://www.openbsd.org/papers/hackfest2014-arc4random/mgp00...