<?php

/* Create array for product list html */
$products_list_html = array();

/* Cycle the items inside the order */
foreach($order->get_items('line_item') as $item) {
    $product_id = (true === isset($item['variation_id'])) ? $item['variation_id'] : $item['product_id'];
    $products_list_html[] = '<a href="' . get_permalink($product_id) . '">' . get_the_title($product_id) . '</a>';
}
echo implode('<br />', $products_list_html);