I already have a copy of Windows 10 Pro which I use in a virtual machine, but I need another Windows 10 virtual machine. Would I be OK to use an OEM version of Windows 10 or do I need to have the retail version?
What's the reason? As you probably be better off using the trial versions in a VM.
You can use any version. You can even use the same Win10Pro disk that served your first VM, if you wish.
I already have a copy of Windows 10 Pro which I use in a virtual machine, but I need another Windows 10 virtual machine. Would I be OK to use an OEM version of Windows 10 or do I need to have the retail version?