retrieve the order Id on Order pay page
global $wp;
if ( isset($wp->query_vars['order-pay']) && absint($wp->query_vars['order-pay']) > 0 ) {
$order_id = absint($wp->query_vars['order-pay']); // The order ID
$order = wc_get_order( $order_id ); // Get the WC_Order Object instance
}