However, there's no information about Android Oreo update hitting these two devices yet. But, Thanks to the leaked HydrogenOS Firmware, you can now Install Android 8.0 Oreo update on your OnePlus 3T.
Since this is an Unreleased Chinese Firmware, we won't recommend you to Install it on your OnePlus 3T. This Firmware is for their Chinese customers with Chinese and English language support. The Firmware has no Google apps included, but you can download and flash it manually.
You can download the Firmware from XDA, and follow this Guide on How to Install Firmware on OnePlus 3/OnePlus 3T. We'll update this Article with Screenshots pretty soon, Stay tuned to our Facebook and Twitter page to get the first News.