I did exactly in your sequence (Open Beta 26) and finally restarted the phone.
Its now stuck (?) on the bootlogo, black background with the oneplus-logo (powered by android)
Nothing happens, since 30 mins approximately.
I repeated the flash (could re-enter TWRP thank god) and end with the same result.
Any help appreciated !
As Vivek posted, do not reboot the device in between. After updating the recovery, reboot to recovery and install SuperSU.
Once you start the steps mentioned, reboot to system only after completing the whole steps.