User blog comment:BertH/New VisualEditor now available in Labs/@comment-3328029-20131210091119/@comment-5558012-20131210202147

"@3 You still cannot use source mode as your default editor while also being able to use the visual; new or old."

You can make source default on the rich text editor and still be able to access source mode with AdvancedOasisUI. I don't believe it works for the new editor.