In Commerce 2, how can I load an order item's 'purchased entity' object given its ID?
If your $order_item is an instance of the OrderItem class you should be able to do:

$product_variation = $order_item-&gt;getPurchasedEntity();
$title = $product_variation-&gt;getTitle();


If you have an $order you should be able to do:

foreach ($order-&gt;getItems() as $order_item) {
  $product_variation = $order_item-&gt;getPurchasedEntity();
  $title = $product_variation-&gt;getTitle();
}