You can actually try installing OpenBSD to a USB drive or external USB/SDCard adaptor first and see if it runs to your liking.
When you trash the hard disk make sure you pick WHOLE disk and then edit partitions accordingly, i do not reacall having problems booting the hard disk up after installing, but you never know ...
I think you hold down the APPLE (command) key to show the boot menu or maybe SHIFT + APPLE keys?
http://guides.macrumors.com/Keyboard_shortcuts
*IF* you want to try a boot manager try this guy out
http://refit.sourceforge.net/