How to add custom checkout field in user details mail template

Use woocommerce_email_customer_details.

function email_order_user_meta( $order, $sent_to_admin, $plain_text ) {
   $meta = get_post_meta($order->ID, 'OIB');
   if($meta){
       /*** whatever format you like here ***/
       echo '<tr>' . $meta '</tr>';
   }
}
add_action('woocommerce_email_customer_details', 'email_order_user_meta', 30, 3 );