Branded PC like HP/Lenovo used to set recovery partition[Windows 10/7], probably with drive letter D/E/F etc. Sometimes there are applications which always work with specific drives. In such cases when you try to change OEM partition, the effort may fail.
OEM partition is designed for system recovery or factory restore. It allows users to easily and quickly restore the system to the original state when system failure or system crash occurs
I personally suggest removing the drive letter from the OEM part, this becomes another troublesome job indeed. With DiskPart command you can remove drive letter of volumes.
Enter cmd command(Window + R) and Run button
Type diskpart on your command prompt
Type List Volume
Choose the OEM voliume with select Volume [volume#]
Execute Remove letter [Drive Letter]
If you are too lucky the OEM drive will disappear from My computer/My Pc. and the drive letter is available for use.