OnePlus has started updating its two flagships devices to latest software update. Now you can download official Android 8.1 Oreo update for OnePlus 5t and OnePlus 5 right through OTA. This Android 8.1 OTA is based on Oxygen OS 5.1.0 and it includes security patch too. It is not very large in size, it is just 766MB in weight. This new update brings many enhancements including improved power settings for having more battery life and speed. The latest OnePlus 5t is getting whole-display gesture for batter usage. And the OnePlus 5 is sadly not capable to receive this.
Keep in mind that the OTA is rolling out by regions, so it is possible you cannot download the OTA quickly. But we hope the company will cover major phases very soon in near future. I recommend you to check for the update manually. Just navigate to settings and there check for software update, if there is OxygenOS 5.1.0 notification to get you on latest version quickly.