We have a number of the GMAT OG 13 explanations that you can find here! Unfortunately, we do not yet have all of the explanation videos for this edition. We have some of the Math practice questions and all of the Sentence Correction practice questions. We're still working on providing all of the explanations!
In the meantime, if you can't find an explanation for a question that you're working on, I would suggest checking out some of the other forums on the web that regularly post help and solutions for OG questions. Here's a blog post Mike wrote about navigating these outside forums: GMAT Forums.