As far as I know there is no official Win 8 iso download from MS that works with OEM keys.
However, the media creation tool for Win 8.1 will work with OEM keys:
http://windows.microsoft.com/en-CA/windows-8/create-reset-refresh-media
You need to you choose the correct edition (you may need to try a few of them if you don't know which edition you're licensed for). I would try the regular edition (simply "Windows 8.1") or the Single Language edition if that doesn't work.
For more details, see this great writeup:
http://dellwindowsreinstallationguide.com/download-microsoft-windows-and-office/download-microsoft-windows/download-windows-8-1-retail-and-oem-iso/
Once you have Win 8.1 installed and updates applied you should then be able to upgrade to Win 10. Warning: do not do a clean install of Win 10 right away, you need to do an upgrade and wait for it to activate before you do a clean reinstall.