"OEM serial key online" is falsely labeled and/or bogus.
If you were to research at Microsoft, you'll find no mention of an OEM license that you can buy on its own.
People upgraded from 7 to 10 because it was Free and it was easy.
But, whatever.
My personal opinion...whatever comes after Win 10 will not incur any direct cost to the user. Unlike previous upgrades.
OEM licenses are the licenses you get when you buy a new pre-built system which includes Win10--they expire along with the motherboard in that system. Stop using that motherboard--Windows license dies along with the original mobo. Additionally, if you upgraded from an OEM license in W7 or Vista or 8/.1 to Win10, your free upgrade is also OEM--and will expire with your current motherboard.
Retail license: You buy these separate from any pre-built computer, and the license follows the lifetime of the purchaser, not the motherboard. I upgraded in 2015 from a retail license for Win7--free upgrade--and my original retail license carried into my Win10 license as a retail license--I've changed motherboards
4 times, CPUs
3 times since then--in every instance Microsoft reactivated the license for the new hardware. (It's not a difficult process.)
As mentioned in my original post, I agree that the next version of Win10, whatever it's called, will likely follow the same upgrade path.
If you want to find out which license you have, do an
Admin command prompt C:\windows\system32>slmgr /dlv and it will pop up a large box that will say either RETAIL or OEM license, and other info. You can also go
C:\Windows\system32>slmgr /xpr and it will pop up a much smaller box giving you the status of your current license.