As each video element wrapped in .ossnembed<guid> class so you mean use below example to change CSS for videos.

<?php
function my_component_init(){
		ossn_add_hook('embed', 'video:css', 'my_component_hook');
}
function my_component_hook($hook, $type, $videcss = "", $params){
		//$params['guid'] post guid,  width , height.
		error_log(print_r($params, true));
		$videocss = "<style>.ossnembed{$params['guid']}{some text here}</style>";
		return $videocss;
}
ossn_register_callback('ossn', 'init', 'my_component_init');