These are the instructions to unlock bootloader on OnePlus 2 smartphone using the fastboot tools on windows computer.
Requirements: Your device should have at least 20 percent or above battery to perform the unlocking process.
(Developer Options is hidden in Android 4.2.2 or above, to show this option: Go to Settings > About phone > Built Number (tap on it for 5-8 times).
(from the above command you will be abe to see the list of devices attached to the computer)
Congratulations, You have successfully unlocked the bootloader of your OnePlus 2 smartphone.
[*] Caution: Unlocking the bootloader will delete your personal data, pictures, apps etc from your device. So, take a backup of your personal data before following the above tutorial.
[*] Root OnePlus 2: If you want to root your OnePlus 2 smartphone then follow How to root OnePlus 2 tutorial.
[*] OnePlus Stock Firmware: If you are looking for the original firmware for your device, then head over to Download OnePlus Stock Firmware page.
[*] OnePlus USB Driver: If you are looking for the original USB Driver for your device, then head over to Download OnePlus USB Driver page.