Yes, it’s possible, but you’ll have to do a bit of string matching to find the id. For example:
$video = get_field( 'oembed_video' );
preg_match('/src="(.+?)"/', $video, $matches_url );
$src = $matches_url[1];
preg_match('/embed(.*?)?feature/', $src, $matches_id );
$id = $matches_id[1];
$id = str_replace( str_split( '?/' ), '', $id );
var_dump( $id );
And then to grab the thumbnail:
<img src="http://img.youtube.com/vi/<?php echo $id; ?>/mqdefault.jpg">
More info on YouTube thumbnails:
http://stackoverflow.com/questions/2068344/how-do-i-get-a-youtube-video-thumbnail-from-the-youtube-api
