Try clearing the app data of the Play Store and then restating your phone.
When in the quick settings menu hold on the gear icon until it starts spinning. The rest is self-explanatory.
Settings > Buttons > Double click power for camera
If it's not available it's because of the Indian government.
Okay. Then here are the steps:
Download the latest Open Beta .zip from downloads.oneplus.net onto your computer.
Boot your phone into recovery then...
No problem! I would really appreciate it if you selected my answer as the best answer. :)