Could you please open a command prompt and type "tnsping" and check for the output? It displays the Oracle client bit-mode. If it shows 32-bit then we need to update the PATH environment variable to 64-bit Oracle client.
In PATH environment variable, please make sure that <ORACLE_HOME>/bin;<ORACLE_HOME>/lib present in the begining. Restart the Informatica Sever and try to start the Repository Service.
Thanks gnagraja.. It worked now. 64 bit client was installed but some of the variables were not properly set in PATH directory. Also, we need to restart the Informatica services again to make use of the variables set earlier.