OnePlus is busy with community builds of OxygenOS that will eventually merge it with the code base of HydrogenOS, but work has not stopped on the stable version of the software. OxygenOS 3.2.6 (still Marshmallow) is official for the OnePlus 3. This OTA brings a lot of small improvements and bug fixes, and as usual, it's rolling out in waves.
Here's the full changelog provided by OP.
Optimized voice call quality of some IM applications.
Improved adaptive brightness performance
Improved camera quality
Fixed occasional green/yellow issue
Optimized white balance
Improved 1080p video recording
Addressed some bluetooth connection problems for vehicles
Allows third party application to modify ringtone
Optimized battery consumption when scrolling
Optimized touch accuracy when playing games
Added DozeMode switch in Developer options
Added PocketMode switch in Display settings
The improvements to the camera are appreciated, but the camera was already quite solid. Although, the white balance didn't see quite right to me in the past. Hopefully it'll be better now. The addition of a Doze mode toggle is interesting as well. The OP3 has very limited software features for Doze because of the alert slider. This might make activating Doze easier in software.
The update is rolling out to a limited number of users today, then it will expand tomorrow assuming things don't explode. The update will be small—just 42MB—when it arrives. Some people in the OnePlus forum thread have posted what appears to be the OTA ZIP, if you're impatient.