Happy New Year OnePlus 3 users. Just before the clock hit the midnight on the 31st December 2016, Tony L. from OnePlus posted on OnePlus forums that the company is rolling out the stable official Android N update for both OnePlus 3 and OnePlus 3T. As always, the update is rolled out on an incremental basis and will reach your device via an OTA update. Apart from Android Nougat, this new update also brings Oxygen OS 4.0 which will be visually a little different if you are a OnePlus 3 user running OxygenOS 3.2.8 or less. Nevertheless, we couldn’t wait and managed to flash the OTA on our OnePlus 3. So here is the guide for you to do the same. 
Note: The guide involves wiping your Data and Cache, so we recommend you backup all your data including apps, contacts, pictures, music etc. to keep yourself covered if something goes wrong. However, it is not necessary to wipe data and cache on some versions but it is highly recommended to do so as this OTA update will bring in a new Android version. The guide includes two methods to flash the OTA update.
Guide 1: Flash via ADB Sideload (Clean Flash)
Step 1 – Download the OTA update for OnePlus 3 from here. For OnePlus 3T, click here. Download the necessary ADB files from here. Note – Run the Minimal ADB and Fastboot.exe
Step 2 – After downloading and installing the Minimal ADB and Fastboot, place the OTA update.zip in the Minimal ADB and Fastboot folder and rename it to ‘update.zip’.
Step 3 – We hope you have already backed up your data on your PC and can now go ahead and Factory Reset your OnePlus 3/3T. You can do this through Stock Recovery. For that, Power Off your device and once powered off, hold the ‘Power Button’ + ‘Volume Down’ simultaneously till you enter the Stock Recovery. Choose ‘English’.
Step 4 – Next, choose Wipe Cache and Wipe Data in order to Factory Reset. We really hope you had your data backed up.
Step 5 – Now, Go back to the main menu of Stock Recovery and Choose ‘Install from ADB’ > ‘OK’ and you will see a screen showing ‘You are in sideload mode’
Step 6 – Connect your device to PC via USB cable and open the Minimal ADB and Fastboot folder. Hold Shift Key and Right Click and Choose ‘Open command window here’.
Step 7 – Once the command window is opened, type in the command – adb sideload update.zip. For Mac users, it is – ./adb sideload update.zip
(Note – We renamed the OTA update file to update.zip in previous steps)
Step 8 – The system will now flash the OTA update. Once done, choose Reboot and enjoy Android Nougat on your OnePlus 3 or OnePlus 3T.
Guide 2: Flash via Internal Storage (Clean Flash)
Now if you found the previous method a bit difficult, this one will help you out. However, this guide is strictly for the devices which have the option ‘Install from Internal Storage or Local’ in their Stock Recovery. We couldn’t find this option on our OnePlus 3 running Oxygen OS 3.2.8.
Step 1 – Download the OTA update zip and place it anywhere in the internal storage of your OnePlus 3/3T device. Download link available in the guide mentioned above.
Step 2 – Power Off your device and enter the Stock Recovery while holding Power Button + Volume Down simultaneously.
Step 3 – Choose English and then go ahead and Wipe Data and Cache just as above. If you have a lot of data installed, this might take some time.
Step 4 – Go back to Recovery Home menu and choose ‘Install from Internal Storage’ or ‘Install from Local’.
Step 5 – Choose the OTA zip installed (Step 1) on your device while navigating through the internal storage and click on ‘Install’.
Step 6 – The device will automatically install the Android N update. Once done, hit ‘Reboot’ and enjoy.
Important Information – You can dirty flash the OTA update by skipping the Step 4 and Step 3 in the Guide 1 and Guide 2 respectively. However, as of now, the dirty flash feature seems to be working only on the Open Beta 10. Further, if you can wait and can’t do the labour to back up and restore your data, we recommend you to wait for the OTA update and install when you get the notification. To check for the update, Go to Settings > System Updates > Check for Updates.