$defaults = array(
      'numberposts'      => -1,
    'post_type'        => 'agent_listing',
      'meta_key'           => 'oepl_order_no',
      'orderby'          => 'meta_value_num',
      'order'            => 'ASC',
      'suppress_filters' => false
);

'orderby' => 'meta_value_num', // for int field
'orderby' => 'meta_value', // text field