OnePlus has finally started rolling out the Android 8.0 Oreo update for the OnePlus 5. This comes as a part of the OxygenOS 5.0 update.
OnePlus has provided the full list of changes on its forum but to summarize them, you are getting all the new Oreo features, along with improvements to the launcher, updated Camera app UI, optimizations to image quality, beauty effect for Portrait Mode, Parallel Apps feature, new design for Quick Settings, new design for lift-up display and more. Also added is the Android December security patch.
The update is now rolling to all the countries, starting with Canada and Germany. You can check for updates from Settings, try using a VPN if it's not available in your region or grab the update from the forum.