How do I find my system serial number and what does to be filled by O.E.M mean

Custom built computers don't have a single, overarching serial number like pre-built systems from major manufacturers. Each component will have its own serial number. If it is the motherboard's serial number you need, sometimes it is printed on the motherboard itself, or see if you can find it in BIOS.

Or see if this command might work.. and that is - for motherboard only.

wmic baseboard get product,Manufacturer,version,serialnumber

I have been trying to figure out how to find my computers serial number because i need to activate windows on my new motherboard but every time I try to find it, it only says to be filled by O.E.M

O.E.M. means Original Equipment Manufacturer.

What I think is happening, here, is that the OP purchased a pre-fabricated (HP or Dell or whatever) PC from somebody, then swapped out the Motherboard to turn it into "a custom" PC. What OP really did was remove the BIOS which contained the O.E.M. embedded license key for Windows.

The new motherboard may or may not even have the ability to embed a license key in this manner, and probably does not. If it did, the only way to get back the old license key would be to reinstall the old motherboard and then there's maybe a 2% chance that the OEM license key for Windows can be viewed and written down for manual input, maybe through that BIOS.

Hello, this is Emily.

Is this a custom built computer? If so, custom built computers typically do not have a single serial number because they are assembled from various components.

If you have a new motherboard, what type of license are you trying to use here? How did you obtain your license? For example, is it from third party, or from Microsoft?

If the previous owner said it was a retail license, he would have to give you the product key. Did he do that? I would not think it would be a digital key since that would have to be transfer to your account from his.

This can be a tough situation - typically you can contact Microsoft support to re-issue a product key, but you need proof of license purchase.

Hello

This computer is a custom build that i bought but i upgraded the MOBO myself, i had windows before but i do not know how he installed it but it said that it was a retail license