Oxygen Tree Opener plugin
I’ve built a simple plugin that automatically clicks on the button that toggles the Structure panel in the Oxygen editor when the editor loads. So essentially all it does it save you one click. In my opinion, the Structure panel should be opened by default.
If the plugin is not working for you, try editing wp-content/plugins/oxygen-tree-opener/assets/js/main.js and increasing the timeout value from 1500 to something like 5000.