OnePlus has already confirmed that OnePlus 3 and the OnePlus 3T will get Android Oreo and finally it is about to happen. The company has rolled out Android 8 Oreo update Open Beta for the same, but as of now you will need to manually install this update.
Open Beta means open for user testing but there are known issues or there could be some issues. This is not the final version and hence this update as of now is not available via OTA (over the Air), instead user will need to download and manually install the update.
If you know what you are doing then you can proceed with the update and be the first to experience the Android Oreo on OnePlus 3 / OnePlus 3T, but if you are not technically sound then ignore it and wait for the final release that could happen within few months and that update will be OTA (Over the Air).
With Android 8.0 your OnePlus 3 and OnePlus 3T device will get lot of features and improvements. You can check all that you will get in Android O and how to install this update on your OnePlus 3T / 3 smartphone from OnePlus official forums.