GetACPI-OEMKey.PS1

$SFTCode = @"

[DllImport("kernel32")] public static extern uint EnumSystemFirmwareTables (uint FirmwareTableProviderSignature, IntPtr pFirmwareTableBuffer, uint BufferSize);
[DllImport("kernel32")] public static extern uint GetSystemFirmwareTable   (uint FirmwareTableProviderSignature, uint FimrwareTableID, IntPtr pFirmwareTableBuffer, uint BufferSize);
 
"@
 
$SFT = Add-Type -MemberDefinition $SFTCode -Name "SFTKlasse" -Language CSharp -UsingNamespace "System.Reflection", "System.Diagnostics", "System.Collections.Generic" -PassThru
 
# 0×41435049=ACPI ? https://github.com/michaelforney/coreboot/blob/master/src/include/cbmem.h
$firmwareTableProviderSignature = 0x41435049
$StructSize = $SFT::EnumSystemFirmwareTables($firmwareTableProviderSignature, [IntPtr]::Zero, 0)
try
{
    $StructPtr = [Runtime.InteropServices.Marshal]::AllocHGlobal($StructSize)
}
catch [OutOfMemoryException]
{
    throw Error[0]
}
 
$buffer = New-Object Byte[]($StructSize)
$SFT::EnumSystemFirmwareTables($firmwareTableProviderSignature, $StructPtr, $StructSize)
[Runtime.InteropServices.Marshal]::Copy($StructPtr, $buffer, 0, $StructSize)
[Runtime.InteropServices.Marshal]::FreeHGlobal($StructPtr)
 
if (([System.Text.Encoding]::ASCII).GetString($buffer).Contains("MSDM"))
{
    $firmwareTableMSDMID = 0x4d44534d
 
    $StructSize = $SFT::GetSystemFirmwareTable($firmwareTableProviderSignature, $firmwareTableMSDMID, [IntPtr]::Zero, 0)
    try
    {
        $StructPtr = [Runtime.InteropServices.Marshal]::AllocHGlobal($StructSize)
    }
    catch [OutOfMemoryException]
    {
        throw Error[0]
    }
 
    $buffer = New-Object Byte[]($StructSize)
    $SFT::GetSystemFirmwareTable($firmwareTableProviderSignature, $firmwareTableMSDMID, $StructPtr, $StructSize)
    [Runtime.InteropServices.Marshal]::Copy($StructPtr, $buffer, 0, $StructSize)
    [Runtime.InteropServices.Marshal]::FreeHGlobal($StructPtr)
 
    $encoding = [System.Text.Encoding]::GetEncoding(0x4e4)
 
    $key = $encoding.GetString($buffer, 56, 29)
}
$key 

Thanks, really interesting script :) It helped me confirm that you can also get this information in CIM/WMI:
PS C:\temp> Get-CimInstance -Class SoftwareLicensingService -Property OA3xOriginalProductKey | Select-Object OA3xOriginalProductKey

Maybe a bit easier to use :)