Root Guide Needed for Oxygen OS 3.1.0

I had tried rooting my device on a previous version of oxygen OS and failed to root as when i flashed superuser.zip through twrp my device got stuck at bootanimation. I installed stock rom after that while my bootloader still unlocked and twrp installed. When i installed the Oxygen OS 3.1.0 OTA update, i got back to stock recovery. I don't know much about this stuff so I'm nor sure if my bootloader is still unlocked.
Can someone provide me with a guide on how to root oneplus two with oxygenos 3.1.0 please.

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