Hi Srikanth,
No it is not possible to use separately installed oracle database and then start the SAP installation.
Oracle installation is integrated as part of SAP installation and you need to give all inputs such as oracle media, client media along with kernel and all Java software DVDs.
Then post preparation and extraction of media, SAPINST will ask you to proceed with Oracle software installation. You can not simply install first oracle software and then install SAP.
From your post, I could see that your client is giving you SID of database and that should not be a problem if they are just proposing you only the DBSID and actually they are not giving any pre-installed Oracle software.
Cheers !!!
Ashish