You can use it with a machine and once activated, it is bound to that machine only. You may reinstall or clean install using the key in that machine as many times you like, but not to a new machine.
ie, once the the key is activated with the new machine, its illegal to use it on your old Windows 8 machine.
OEM: Original Equipment Manufacturer, can be defined as entity or person who makes or assembles the PC....
For installing on a different PC, you will need the retail version, not OEM one.