Oneplus 3 and 3T are receiving Oreo update via the Open Beta builds. Oneplus 3 is being updated to Oreo via OpenBeta 25 and 3T via OpenBeta 16.
Those who are already on beta builds can get the updates via OTA. Devices that are on stable version can clean install openbeta full rom from the official oneplus website. Or else, they can wait some days for stable oreo update via OTA.
Some screenshots:
Update Log
September security patch
Oreo Easter Egg
I have not yet found anything wrong with the update for the last 10 hours I'm running it. It has got a nice Octopus easter egg!