Using the media creation tool, you either have to of had windows already on it (Windows 7/8/8.1 activated or a product key for Windows 10. If there is a previous activated windows on it, just skip the key, when its done then it gets on line and will automatically activate
When you are using the tool, and there is already a version of windows, disconnect from the internet before trying to install. After done then connect and it will do the updates.
An OEM license refers to the license that a manufacturer installs on new devices. If this is your case, the product key isn't transferable, and you can't use it to activate another installation. (Unless you're re-activating a new installation on the same computer.)
Unfortunately if you do not have a Windows 1o key you will need to buy one. There are plenty of Legit sites you can get a key that works relatively cheaply. Just do a google search and be sure to read the reviews first.
One of the sites I have used several times is:
https://microsoftsoftwares.com/