As a part of an effort to prevent the spread of misinformation and false news, Facebook removed the ability to edit link previews (i.e. headline, description, image) on July 18, 2017.
Whenever you share a link from Oktopost to Facebook, Oktopost will attempt to retrieve the link preview and display it based on the content of the page you are trying to share. If Open Graph Meta Tags are present on the page, there's a high chance that Oktopost will use them. If not, Oktopost will attempt to fetch the link preview directly from the content of the page.
Since Oktopost and Facebook use different engines to parse link previews, discrepancies can happen. The best way to ensure that your content is shared correctly is to make sure that your Open Graph Meta Tags are set correctly using the Facebook Sharing Debugger.