The ADO (SQL Connect) Monitor will simply connect to your SQL server installation, login and then disconnect. On the other hand, the ADO - User Experience (SQL Query) Monitor will connect, login, perform the specified query, expect specified results and disconnect. Having that said, as you are getting an error such as "Could not locate any tables", I would presum that the Credential you have assigned to the Monitors is a valid Oracle account, however, it does not appear to have permission to the database you are attempting to query. Having that said, I recommend reviewing the security settings on that account in order to ensure that it has at least read access to the database in question.
Hope this helps.