Based on the documentation, namely
Checking Autogenerate Descriptions will mean that All in One SEO Pack will automatically generate your OG:Descriptions for each Page or Post from the Excerpt, if you create one, or from the first 160 characters of your content if you do not create an Excerpt.
My expectation would be that the og:description tag would be populated with the content of the manually-set excerpt for a post, if there is one. This doesn't seem to be happening on my client's site, and indeed the code doesn't seem to use the post_excerpt at all. In aioseop_opengraph.php, method All_in_One_SEO_Pack_Opengraph::add_meta, a truncated post_content is used to set the description rather than the post_excerpt.
Replacing post_content with post_excerpt in that method does cause the description to be populated with the excerpt, though of course that change on its own doesn't provide a fallback if there's no excerpt.
Anyway, just wondering if I'm missing something, or if I'm misreading the documentation, or this is a bug.