The most common reason for the ORA-27123 error is that you have not correctly configured your kernel to support the Shared Memory Segments that Oracle requires on Unix/Linux platforms. The Oracle Installation Guide fully documents the kernel changes you need to make in order to fix your problem. You can find these steps here.
In the /proc/sys/kernel directory, you'll need to modify the shmmax, shmmni and shmall config files as described in the above documentation.
This was first published in February 2006