Referred the tableau online help and made note that Tableau Online cannot support direct connections to Oracle data hosted in the cloud.
I am still not convinced that Tableau not supports oracle, I will go back to Tableau and ask them if there any plan to bring this feature on an oncoming release ?
I do not have any update from Tableau yet but I should able to share the outcome of my testing.
Tableau desktop connects to the Amazon RDS for oracle - Selected Oracle data source while creating a new workbook and then given my Oracle RDS details on the connection parameters. I am able to connect and create a new workbook.
Tableau online connects to the Amazon RDS for oracle through Tableau Bridge services and refresh the workbooks.