Autocomplete
This feature is disabled. By default. You will need to enable it per builder.
Top bar > Settings > Enable Autocomplete > Builder

Learn more about autocomplete
Autocomplete Location
The Plain Classes field is located at the top of the left or right sidebar. Oxygen have option to set up sidebar location position. It becomes visible once you select an element inside the Oxygen iframe or from the list tree.

Compile on post save
With the Builders integration, we also implemented automatic compile-on-post-save. You no longer need to open Winden manually to recompile your CSS. Whenever a post is saved, a signal is sent to Winden, which scans all classes used in the post and regenerates output.css automatically.