WooCommerce - add prefix to order number for specific users

add_filter( 'woocommerce_order_number', 'arglebargle_order_number' );
function arglebargle_order_number( $order_id ) {
    $o = new WC_Order( $order_id );
    $usr = new WP_User( $o->get_customer_id() );

    if( in_array( 'administrator',  $usr->roles ) ) {
        return ( 'ARGLE-' . $order_id ) ;
    } else {
        return ( $order_id );
    }
}