delete OEM system partition win 10

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.
Be careful before you decide to run the commands, lest you mess up – you should know what you are doing:

• 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.

If you want only to merge part of the partition, then use extend [size=]  command. To extend size by 5GB, type-

Extend size=5000

Here size is the size you choose from the OEM partition.  It will extend the selected volume by size in megabytes (MB). So like you can see, delete and merge partition commands work hand to hand. You need first to delete it and then merge it existing partition.