I put it at the bottom of the file and it works perfectly! Thank you so much :D

Here is a component: you can move the code into your ossn_com.php file

function forum_menu_priority() {
		if(com_is_active('Forum')){
			ossn_unregister_menu_item('forum', 'links', 'newsfeed');
			ossn_register_sections_menu('newsfeed', array(
					'name' => 'forum',
					'text' => ossn_print('forum'),
					'url' => ossn_site_url('forum/categories'),
					'section' => 'links',
					'icon' => true,
					'priority' => 1,
			));					
		}
}
ossn_register_callback('ossn', 'init', 'forum_menu_priority');

Aha. Is there an easy way to do that? If yes, how do I do that?

I believe priority won't work here, you have to use javascript and move the menu item link to forum to top of the newsfeed item.