How to get an array out of a nav menu if it’s a plugin?

What you have is a theme location, you need to first get the menu assigned to that location to get its items:

if( ( $locations = get_nav_menu_locations() )
    && isset( $locations[ 'omahamenu' ] ) ) {
        $menuooo = wp_get_nav_menu_items( $locations['omahamenu'] );
        var_dump( $menuooo );
}