I ran into the same issue when i was installing the new Dot 3 using 11gR2
if you send me your Emnail I will send the details and the screen shots on how I fix the same Issue
you have to set environement variables $ORACLE_HOME and $PATH in user profile ( .bash_profile in your home directory). some think like this :
after that you execute ths profile . .profile or reconnect to load the variables, you can then use netca,sqlplus or any other utilities in bin directory of your ORACLE_HOME
If you type in console dbca (for example), it can be successfully executed only if you added your ORACLE_HOME/bin to your path(bash_profile file in your home directory).
Other way to execute dbca(or any other) command is to type full path to dbca(or other) executable file.
For example /home/oracle/u01/app/product/11.2.0/dbhome_1/bin/dbca.
If you set environment variables (PATH , ORACLE_HOME, ORACLE_SID , LD_LIBRARY_PATH) correctly , you also should set DISPLAY variable in order to run gui based applications like (netca , dbca ... ).If you are using vnc, just set DISPLAY=hostname:0.0 . If you are using XManager ,set DISPLAY=your_machine_ip:0.0 and also check "xhost +"