Essential announced that it would be skipping a full release of Android 8.0 Oreo, instead going straight to Android 8.1 Oreo. We're about a month in since the first beta build of Android 8.1 Oreo came out for the Essential Phone, the official Android 8.1 Oreo update has been released.
The full Android 8.1 Oreo update comes in at around 276MB, though that'll vary depending on whether you're on the beta or not, it's around 1,015MB if you're coming from Android 7.1 Nougat.
The changelog mentions standard Oreo features such as Instant Apps, picture in picture, smart text selection, and more. Essential is adding the March security update, dynamic calendar and clock icons, and an apparent fix for the slow scrolling jitter.
Source: Essential on Twitter