OnePlus has now announced a new software update for OnePlus 3 and 3T that isn't Oxygen OS, but the Hydrogen OS 3.0. The new operating system is now available for both devices and is based on Android 7.0 Nougat with various changes over the Oxygen OS.
Although the build is for the Chinese version of OnePlus 3 and 3T, you can install it for yourself from recovery. There are no Google apps pre-installed and the ROM is 1.2 GB in size. According to users and first tests, the new firmware has slightly better battery life compared to the latest Nougat OxygenOS update.
If you want to test the firmware, check the official OnePlus page.