SSIS is needed because BPC Packages are built on top of SSIS components. BPC Package is SSIS package in natural.
From what I can see so far, your financial system can continue to run on Oracle.
BPC and its associated SQL server can be installed on a server(s) separated from Oracle. You then build and run BPC Packages which will pull data from Oracle onto SQL server via an OLEDB connection.
Thus I don't see why you need to migrate from Oracle to SQL yet. Perhaps you can tell us why you think a migration is necessary.