I have installed the Oracle client but I am not able to install the Oracle server. How do I install Oracle server?
Without knowing for sure what your exact problem is, I can tell you that I have successfully installed Oracle EE on Ubuntu. There are several steps involved in this process, so I'm going to take an educated guess at what might be holding you up.
Ubuntu is not on Oracle's list of officially-supported Linux distributions, and the installer is smart enough to figure that out. To get past that, you need to add a flag that tells the installer to ignore the system pre-requisite checks.
If you're running into additional problems, please post your exact error messages, or check this step-by-step process on how to install Oracle 11g R2 on ubuntu 12.04. You also might want to check this question: How can I install Oracle 11g R2?