Dividing Memory between Oracle 11gR2 and RHEL 6.5

Hi,

I have a dedicated server running RHEL 6.5 with 256 GB of RAM and I want it to run Oracle 11g R2. However I don't know how much RAM should I leave for the OS and how much for Oracle. The default installation creates /dev/shm with half the RAM (128GB) but seems a waste to leave the 128 GB for the O/S. I couldn't find any official recommendations from RedHat regarding this.

Cheers.