Hi,
Its not possible to use a separate installed Oracle database for SAP Installation. Please Read the SAP Installation Master Guide where it clearly states Oracle Software provided is integrated with SAP Schema.
http://help.sap.com/bp_ekit604/documentation/291009_EhP03_MG.pdf
One thing u can do is Export and Import the database once the SAP Installation is complete.
Regards,
Sandeep Singh