Rational build forge setup instructions guided me to install thin client to have rational tools working with oracle , jumped to install "instant client" which was nothing but extracting a zip file which server did not like and it kept cribbing about error stack with any of following error->
Error 6 Initializing SQL*plus
SP2-0750: You may need to set ORACLE_HOME to your oracle software directory.
message file sp1<lang>.msb not found.
Lot of people commented on this issue being one of setting the correct environment variable issue particularly "ORACLE_HOME".
However for me it was more than that, Oracle "instant client" does not have all capabilities that rational server required. So next logical step was to get "SQL plus" running on machine and most appropriate is to install full client rather than "top up" of instant client (options provided by think client downloads) else you will have same issue. Use Oracle's inbuilt installer to install the SQL client as it sets correct environment variables and also sets registry entry.
Once you finish installation via oracle installer and you are able to launch SQL plus client you are all set to go and Rational server will be happy too !