@shayne, the ODD plugin requires changes to the checkout_payment and checkout_confirmation pages ... neither of which are used by the OPC processing.

As you'd indicated, you'll need to make template-override versions of one of the OPC's template sections. To add the ODD information to the comments area, you'll copy /includes/templates/template_default/templates/tpl_modules_opc_comments.php to /includes/templates/your-template-name/templates and make your edits there.