OnePlus is rolling out the Oxygen OS 5.0 based on Android 8.0 Oreo through FOTA for the OnePlus 3 and 3T starting today. Do check for software update on your device.

Here are the features and highlight of the build;

System
Update to Android O (8.0)
Added Parallel Apps
Added Picture in Picture
Added Auto-fill
Added Smart text selection
New Quick Settings design
Updated security patch to September

Launcher
Added notification dots
New app folder design
Now able to upload photos directly to Shot on OnePlus

Source: OnePlus Forum