Oracle XE Fedora 15 Installation problem
Some thoughts:


First, I also have tried to get Oracle XE up and running under F15. Finally managed it, but it wasn't as simple as I would have thought it should be -- It's easier to get the silly thing configured under Ubuntu. IIRC, I just ended up installing EE instead (under F15).
Second, your environment needs to be set up correctly in your .bash_profile (in your home directory). Make sure to export ORACLE_HOME, ORACLE_SID (=XE), and make sure $ORACLE_HOME/bin is exported in your PATH. Once done, log out and back in and you should be able run sqlplus fine. (If you don't want to log out, just run ". ~/.bash_profile", and it should also work.)
Third, I don't know why Oracle doesn't set this up during the install process. And of all users not to have the environment set up (when XE usually has to create the user from scratch)... the oracle user should simply be set up with it from the start. But that's a rant and not really related to your question.


Hope that helps...