Show all Opportunities with filter: compare Parent_Account_ID__c value from Opportunity page (a formula field CASESAFEID(Account.ParentId)) with 2 fields from a dataset: AccountId and Parent_Account_Id__c.
As per Summer '19 release, we can use Filter Builder or Filter String, but unfortunately, the filters do not support OR logic.
Here is the workaround for the scenario:
1. Create a field in Dataset using ComputeExpression to concatenate AccountId + Parent_Account_ID__c, let us say AccountIdnParentId.
2. Use the new field created from ComputeExpression with "Contains" operator.
In case if Parent Account Id is blank, this will cause the dashboard error, you can create a custom formula field in the Opportunity to check and make sure if Parent Account Id is blank, use Account Id.