It's not that simple, an ARM board is not a PC, you don't have a BIOS for instance. Basically you have to write some glue to tell the kernel what and where are the devices and how to access them.
Not to mention that while OpenBSD supports the ARM architecture it might not have drivers for all the IPs integrated in the SoC.
So basically, it won't be plug'n'play like installing a new OS on your new x86 computer, altho the amount of work required to get it to work might not be completely overwhelming if you know what you have to do (especially since the linux source code is available).
See for instance in Linux http://lxr.free-electrons.com/source/arch/arm for the various Board Support Packages available upstream.
Not to mention that while OpenBSD supports the ARM architecture it might not have drivers for all the IPs integrated in the SoC.
So basically, it won't be plug'n'play like installing a new OS on your new x86 computer, altho the amount of work required to get it to work might not be completely overwhelming if you know what you have to do (especially since the linux source code is available).