1. Click on: Setup | Customize | Opportunities | Page Layouts.
2. Click on the "Edit" link next to the page layout name.
3. Select related lists from the drop down list in box on the left hand side of the page.
4. Click on cases to drag and drop it to the related list section.
5. Click "Save".
You will now be able to view all cases related to an account when viewing an opportunity associated with that account.
It is recommended that you add the Opportunity Name to the case related list, so that the user will know which cases are related to the opportunity in question. Read More: Customize Related Lists.