OK I’ll walk you through it. You don’t have a specific OG page. Each page has it’s own data.
Go to the page settings and click on the add code button. A window will pop up and your code goes in there.
From there here code will be edited to look something like this:
<meta property="og:type" content="website"/><meta property="og:title" content="Advanced Website SEO Services"/><meta property="og:image" content="https://example.uk/hotlink-ok/example-Opengraph.jpg"/><meta property="og:url" content="https://example.uk/advanced-seo-services/"/><meta property="og:description" content="Advanced SEO services from Example Web Design that will boost your web ranking"/> <meta property="og:site_name" content="Example Web Design"/> <meta property="fb:admins" content="yourfacebookpageid"/>
Your image should be hosted remotely somewhere on the server without using a bric. I always put them in a folder called hotlink-ok out of habit because I’ve used Cloudflare, which suggests that name for their own services, but you can call it what you like otherwise. The important bit is simply that it is a url full path that takes you directly to the image.
The rest should be fairly easy to work out, but always test it with Facebook developer tools to see how it looks https://developers.facebook.com/tools/debug/
This is a good site for resizing your images to the correct proportions https://sproutsocial.com/landscape
PS I always call the preview image file opengraph or something similar, just so it is easy to separate from twitter images in the same folder when checking on the server.