Cases can't have links to Opportunity Line Items directly. However, there are two fields you can expose, Asset and Product, which allows you to link to a specific product, or an asset, which is an instance of a product that belongs to an account or contact.
Generally speaking, upon winning an opportunity, assets should be created for the account in order to note which products they actually have. This would also allow you to specify things like serial numbers, product, and custom attributes specific to your line of business. Some companies do this manually, some have written code to do this automatically, and some do not track this data at all (but, for most companies, I'd say that they probably should track this information).
In fact, if you are using assets, you can go to an account, locate the appropriate asset, and click the New Case button to automatically start a new case for that specific product. When everything is set up, creating a new case from the asset page will populate the account, contact, asset, and product for you automatically. This can greatly reduce the complexity needed to start a new case. Of course, it would also be possible to use other methods, like a Visualforce page, etc to populate this data, but can definitely be done out of the box with minimal effort required.