OpenBSD-Gentoo dual boot on an iMac

[chmeee]

Hi there,
currently I have my iMac with OpenBSD booting from a rather heretical MS-DOS MBR.
I have just noticed this BSD-like flavour of linux and would like to try it out in that machine, but I don't want to be without OpenBSD.
Is there any way to dual-boot them?
OpenBSD can dual-boot with MacOS, but it needs at least a HFS partition.
So, my question is, as I don't have MacOS, can I share the iMac with this two operating systems?
I know I have to reinstall OpenBSD but I don't really mind.

thanks everyone,

Chema.