I am successfully posting from the OpenBSD 5.6 install I did on this new PC. I will set it up for a dual boot in the coming days.
You really want to install OpenBSD as the second OS in a dual-boot setup. You
can shoehorn a dual boot onto it with a different OS later, it's just more annoying and error-prone. I recommend reading
this and making sure you understand it before attempting a dual boot; the OpenBSD/amd64 boot process is very simple and straightforward (or as simple and straightforward as amd64 gets), but understanding how it works is important for being able to integrate it into some other bootloader.