I have a broken old laptop.
However, it does have one useful thing: a Win7 license key.
Can I use this key to install Win7 on a desktop and upgrade to Win10?
The laptop authenticity sticker on the underside says its "Home Premium OA" and OA I think means its already activated. So I wonder if I have to "deactivate" it first before activating again?
I don't think so. OA I believe simply means it is a factory OEM (Original Equipment Manufacturer) installation and that means the license key is only valid for that PC. If you attempt to use it then you will just get a failure notice as the key is already in use.
I just read somewhere that I have to deactivate a Windows license key, and only then can I re-use it.
slmgr /upk [your-windows-key]
https://www.howtogeek.com/124286/how-to-uninstall-your-windows-product-key-before-you-sell-your-pc/
Doesn't apply to OEM as you can't reuse it anyway and in any case if the laptop is broken you'll not be able to do it.
You could try installing 10 and entering your 7 key (you don't have to install 7 and upgrade). It will either work or not. If it doesn't work you could try telephone activation, tell them it is only installed on one computer and see what they say. As @cereberus said according to the license you can't do this but there is nothing wrong with trying.
OA simply means it's an OEM Activated Product Key. From Win7 back, most times a Key from one brand was prohibited from installing on a different brand, especially when using a CD for installing. As mentioned, Win10 changes things.
Factory Windows 7 OEM installs use a Generic OEM key. Dell has their keys, Acer theirs, etc. As an example every Dell with Windows 7 Home Premium will have the same Dell key. Those PC's don't activate online, they activate against a SLIC table in the BIOS. Use a Dell OEM disk on an Acer and the SLIC table won't match, no activation. No SLIC table means no activation too. Those generic OEM keys are also blocked from online activation. Windows 10 won't activate with them either. That being said, that generic OEM key is not the key printed on the COA sticker. That key is unique and its there for use with stock Windows 7 install media. I've had to do a phone activation when using them though. According to the EULA, OEM keys/installs are not transferable.
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