I made the mistake of deleting Windows 8 OEM Recovery partition instead of just making it inactive.
I used diskpart:
select partition n delete partition
Now I see the unassigned space in the Disk Management utility (but not in the diskpart).
My guess is that I can create a partition in this space (as nothing was done to it yet) and somehow mark that partition as OEM.
Am I right? Either way, how can I restore my OEM partition?