For this example to work you will need to install Advanced Custom Fields, create a text field called "youtube_video_id" and then when you are creating your post, you can embed your video, and then use the ID from YouTube into the text box.

functions.php

add_action('wp_head', 'chillybin_custom_og_image');
function chillybin_custom_og_image() { 
  if (get_field('youtube_video_id')) { ?>
    <meta property="og:image" content="http://img.youtube.com/vi/<? echo get_field('youtube_video_id'); ?>/mqdefault.jpg" />
  <?php }
}