OnePlus recently rolled out OxygenOS 3.2.0 update for OnePlus 3 with new features and fixes, but immediately stopped it due to some reports of issues (critical). Now, the company starts rolling out the new OxygenOS 3.2.1 update for OnePlus 3 by fixing all the issues. If you have already updated to OxygenOS 3.2.0, this update will bring some additional improvements to your device.
The new OxygenOS 3.2.1 update includes all the improvements included in the previous 3.2.0 update, like the sRGB mode (offers more accurate color reproduction) in developer options, improved RAM management (bring faster loading of apps and games), fix for SIM recognition issue, better GPS performance, etc. as well as additional improvements and fixes.
Below are some of the highlights of OxygenOS 3.2.1 update for OnePlus 3
Fixed some notification issues
Addressed SIM recognition issue
Enabled sRGB mode in developer options
Improved RAM management
Improved GPS performance
Enhanced audio playback quality
Updated custom icon packs
Improved camera quality/functionality
Fixed some issues in Gallery
Implemented latest Google security patches
Fixed bugs in Clock/Music apps
This is an incremental update and you do not need to update to the previous OxygenOS 3.2.0 to get the new update. As always, the OTA update is rolled out in phases, so it will take few days to reach the update on all devices.
If you have not received the OTA update notification on your device, you can manually check it by going to phone Settings > About phone.
Manual Installation of OxygenOS 3.2.1
If you are prefer manual installation, then you can download and sideload the ROM zip file via adb. If you have already updated to OxygenOS 3.2.0, just download the OTA file [HERE] (15MB) and install it. If you have not installed the 3.2.0, then download the full ROM [HERE] (1.2GB) and sideload it via ADB. You may follow the flashing instruction as given here (source)