currently produces og:url as schema-relatve, but fully qualified og:image.
|mediawiki/extensions/OpenGraphMeta : master||OpenGraphMeta should provide relative URLs for og:image|
Reading the abandoned change on gerrit, il seems that the OpenGraph protocol does not support relative URLs. Indeed, this is what searching the web for the issue confirms. Though it's not explicit about needing absolute URLs, the specification does not indicate that it can take a relative URL either; neither is Facebook's developer page.
I'm going to show some initiative and decline this one.