Windows Disk Management Tool will not let you delete/merge the OEM partition. You will have to use the built-in command line tool called Diskpart.
- Open the Run prompt, type diskpart, and hit Enter.
- Type, and enter list disk to list the disks.
- Select the disk you want to manage – say it is Disk Z
- Then type select disk z and hit Enter.
- Enter list partition and hit Enter to display all the volumes.
- Type select partition x and hit Enter. Here x stands for the partition you want to delete.
- Finally, type delete partition override and hit Enter to delete it.
- Now type Extend to merge the OEM partition with the adjoining value.