Has answer to this question changed over the past four years?
OAuth is an user authentication and verification mechanism which allows any website to delegate authentication to a third party which you trust like Google, LinkedIn, Facebook or Azure.
You can combine REST with OAuth, that is often done with adding a ticket or token to the REST request. This ticket can be obtained via OAuth. That solution would require an OAuth server implementation for Mendix which is not available yet and not something that can be added easily to the current module.
In case of a Mendix-Mendix connection you can provide a ticket/token with a published web service over SSL and a username/password which is not more secure than your current implemention.