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.

1. Open the Run prompt, type diskpart, and hit Enter.
2. Type, and enter list disk to list the disks.
3. Select the disk you want to manage – say it is Disk Z
4. Then type select disk z and hit Enter.
5. Enter list partition and hit Enter to display all the volumes.
6. Type select partition x and hit Enter. Here x stands for the partition you want to delete.
7. Finally, type delete partition override and hit Enter to delete it.
8. Now type Extend to merge the OEM partition with the adjoining value.