OnePlus has begun rolling out the India-specific build OxygenOS 5.1.8 for the OnePlus 6 sets in the country. The update optimizes the call quality for the Indian users while improving the system and network stability too. The update also includes OxygenOS 5.1.7 improvements, which means it fixes the bootloader security bug for the Indian users.
Meanwhile, both fo the OnePlus 5 and OnePlus 5T are seeing an update to OxygenOS 5.1.3. The OOS 5.1.3 update fixes the issue with OnePlus’s bootloaders, but there is not much other than that. A small issue with cars Bluetooth connection seems to have been fixed with the update, too, BTW.
Check out the pages linked above to find all the news about OnePlus updates for the device, as well as the firmware download link of each update.