Oxygen 4.0.3 corrects a few remaining edge case bugs & issues found in Oxygen 4.0. There were no changes between Oxygen 4.0.3 beta 1 and Oxygen 4.0.3 RC 1.
Existing users can download Oxygen 4.0.3 RC 1 in the customer portal.
Before using a release candidate in a production environment, backup your site. While release candidates have been tested extensively and are considered stable enough for production environments, you should always proceed with caution when using any alpha, beta, or RC version of Oxygen or the associated add-ons. We will provide official support for sites running release candidate versions of Oxygen and the associated add-ons. If you encounter bugs, please report them on GitHub.