Hello KiCheol,
Thank you for posting question on Microsoft Windows Forum.
Well! You can legally use a Windows 11 Pro OEM license on a computer without an operating system, provided the license was purchased from a legitimate source. OEM License which is bound to the hardware (specifically the motherboard) of the first PC it is installed on and cannot be transferred to another PC later.
To further elaborate the terms of legality. Buying an OEM key from a legitimate vendor (Microsoft partner, authorized reseller, or bundled with hardware) is legal. Using it on a PC without an OS is allowed, as OEM keys are often sold with barebones systems or for system builders. What is illegal is buying cheap, suspicious keys from unauthorized sellers (e.g., marketplaces offering “multi-activation” or “lifetime” keys at very low prices.
Since the computer has no OS, you might need to create Windows 11 installation media (using a USB drive). Install Windows 11 Pro. Enter the OEM serial number during or after installation. The PC will activate online. Once activated, the "digital footprint" of that hardware is stored on Microsoft's servers.
Hope the above information is helpful! If it is. Free feel to hit "Accepted" for benefitting others in community having the same query too.