What is the Sql server version ?
How many tables approx ?
What volume of data involved ?
You have few options and all depends on the amount of data you are going to tansfer and the complexcity.
Option 1 : Link SQl server to oracle and can retrieve the SQL server data within oracle.
http://www.dba-oracle.com/t_database_link_sql_server_oracle.htm[
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Option 2 : Can use ETL tools (e.g. Informatica, Cognos)
Option 3 : Can use sql server integration services (SSIS), just do a google and you will find plenty of articles