I did this with a Toshiba Laptop that came with OEM Win8. I got Win 8 PRO and did a fresh installation without having to enter any serial number, the key is to make sure your OEM version is PRO as well, if not, you need to purchase the same version that came with your laptop.
As patcooke said, make sure to run the recovery disk creator because if you have any issues and need to send it in for warrantee repairs, it needs to be stock OEM or they don't have to repair it.
Also make sure to find the drivers folder that came with the OEM install, unless its a separate disk, and copy it to an external so you have all the required drivers handy after your install.