OxygenOS 5.1.0 update starts getting on OnePlus 5 and 5T through OTA, which weighs around 766MB. It seems that devices run on OxygenOS Open Beta channel will get the stable Oreo 8.1 update first. Non-beta participants may need to wait a week or more to get the new update.

What is new in OxygenOS 5.1?

Update OnePlus 5/5T to Android 8.1 Oreo Includes April 2018 Android security patch Full-screen gesture support (Only for 5T) Optimization for Gaming mode, which includes power saving and pausing adaptive brightness Network boost – network priority for gaming App in the foreground Category tags for app drawer search and auto name folders based on category

Since the OTA update roll out in phases, it will take a few days to get the OxygenOS 5.1 update on OnePlus 5 and 5T. To get the OTA update early, you can use a VPN app to download the OTA update. In VPN, change the location to Germany or Canada and check for OTA update. If you prefer manual installation, OxygenOS 5.1.0 full firmware will be available at OnePlus download section. You can install the update manually as described here. (Source: 1, 2)