OnePlus is still plugging away at the merger of Oxygen and Hydrogen OS with a new community build for the OnePlus 3. This time, it's version 3.5.3, and the changes are surprisingly substantial. You can give it a shot now, but the usual warnings about bugs in pre-release software apply.
Here's the changelog provided by OnePlus.
Use AOSP clock as default clock
Added capability to uninstall some preinstalled apps, like FileManager, Weather, Recorder, MusicPlayer
Added expanded screenshots, you can now save a long screenshot of a scrollable page to cover more contents.
Added celluar data firewall, you can control whether an app is allowd to use cellular network or WiFi
Added“Favorite contact setting”in Priority mode
Setting menu adjustment in Messages, added option to vibrate phone when receiving SMS
Contact UI redesign
Fixed community build cannot flash back to official build issue
General bug fixes
There are a lot of little tweaks to the feature set this time. The cellular data firewall is interesting, though I'm not sure "firewall" is the most accurate way to describe this. You will apparently be able to block an app from using any cellular data in the settings. The default apps get some changes too; the clock is now AOSP and other stock apps can be completely uninstalled. Scrolling screenshots a la Samsung are included too (OP calls these "expanded screenshots").
If you want to check out the new build, download the image (1.3GB) and move it to your device. OP says you'll need the stock recovery to install it, but it might work fine in TWRP. No promises, though. You'll get OTAs on Community Builds, but not official ones. If you want to get back on the official builds, you need to wipe data and reflash.