The easy way of adding Oracle XE on Ubuntu. There is now an apt-get repository up on oss.oracle.com for XE.
deb http://oss.oracle.com/debian unstable main non-free
Get the certificate with:
wget http://oss.oracle.com/el4/RPM-GPG-KEY-oracle -O- | sudo apt-key add -
Then open the Synaptic package manager by clicking the menu System > Administration > Synaptic Package Manager. (hint: in Ubuntu to do priviledged things you can use the command sudo from the commandline and enter you normal password when asked to. You can do this on window popups too. Ubuntu is configured so that you never know the root password you just sudo everything and your current password will work. If you want to be root at the command line just type sudo su).
Now in the package manager click the reload button (if you are behind a proxy be sure to set it in Settings > Preferences). Once it is done click search and look for oracle-xe-universal. click the box next to it and select Mark for Installation. When it is done you'll have a nice menu entry under Applications > Oracle Database 10g Express Edition.
I found this info in this post.
Once you have Oracle installed, you need to add your user to the dba group using System > Administration > Users and Groups
It looks like you are done from the installer, but there seems to be one other thing you need to do: sudo /etc/init.d/oracle-xe configure