1. Login to server through oracle user.
2. Give command like cd $ORACLE_HOME/network/admin
3. In that list the file and give cat tnsnames.ora it will show tns file contents
4. Copy and paste in your notepad. (Same steps(1,2,3,4) for all database whatever you want link)
5. Download and install Oracle 11g client in SQL server 2008 database installed system.
11g Client installation screen shot for your reference.
6. Once oracle client installed then go to C:\Oracle\product\11.2.0\network\admin
7. In that location you can see the tnsnames.ora. give full permission to that file and copy/paste tns entry details as you copied before.
8. Finally you can see the Ora.OLEDB.Oracle component in SQL Server 2008 from below mentioned path.
9. Through this component you can access Oracle database objects.