Not intending to make you go in circles
The Open Graph - Content plugin is what is inserting the open graph image tag and making that work. However, the default of the Open Graph - Content plugin is to set the og:description tag to the first words of the article. Knowing that disabling it changes the behavior helps us understand things quite a bit.
I know you mentioned you created a new Custom Object. I think that's the way to go. You'll want to create a new Content - Article object and set it to the "Root" category. In the settings side, set the description to come from the Meta Description. That should do what you're looking for and apply it to all articles on your site. Test though and let us know how that goes.
Thanks,
Alex