There are a few ways to get access to the {order_hash}
tag, the most common way that you would probably see it is a result of placing a successful order. This would be achieved by adding return="order/ORDER_HASH"
to the final checkout tag.
The next way you can access the order_hash is in your receipt templates that will get sent out to customers on a completed order. You will want to use the {exp:store:orders}
tag pair in your receipt templates to get all of the relative order details and this would allow you to create a link to the particular hash (this assumes you have a template group called orders and the template is looking for the hash in the segment_2. You may need to adjust to your site.)
{exp:store:orders}
Order ID: {order_id}
View Invoice: {path="orders/{order_hash}"}
{/exp:store:orders}
The other way you can access the order_hash was touched on by Rodrigo in his answer but you should be able to use the {order_hash}
tag even though it is not listed on the orders documentation page.
{exp:store:orders member_id="CURRENT_USER"}"}
Order ID: {order_id}<br />
<a href='{path="orders/{order_hash}"}'>View Invoice</a>
{/exp:store:orders}
The nice thing about using the order_hash over the order_id as has been mentioned is that it prevents a malicious user from being able to quickly guess other order numbers which might include sensitive information such as address or phone number.
If you do decide to use order_id instead you should make sure that your template to show single orders also includes a conditional for the current member to prevent users from seeing other users orders.
{exp:store:orders member_id="CURRENT_USER"}" order_id="{segment_2}"}
Order ID: {order_id}<br />
{/exp:store:orders}