Update on July 23, 2019
This plugin is now in the WordPress repository here.
Rank Math SEO has recently released an API using which content from other themes and plugins can be set to be analyzed by Rank Math.
I have built a plugin using this API and the code built-in into Oxygen for integration with Yoast.
Using this connector plugin will enable Rank Math to parse text (incl. alt tags of images) inside the Oxygen editor for Pages and Posts.
There are no settings for the plugin. Just install and activate to make Rank Math include Oxygen in its content analysis.
If you would like to see this integration in Oxygen natively, upvote the feature request here.
This plugin has been set to read content from the WordPress editor AND the Oxygen editor for inclusion in Rank Math’s SEO analysis.
Therefore, you are advised to not have the same content in both these places.