Porting from MS SQL Server to Oracle

Hi,
Need help on porting code from MS SQL Server to Oracle. Could u please advice on the requisites for porting ? In our case, SQL Server has stored procedures also. Will there be any extra effort required to port stored procs ? I have not worked on SQL Server. Can the codes written in SQL Server be re-used as is or is any alteration needed ? I would like to know about how constraints are dealt with.

Regards,
Aparna S.