Oracle is an organisation, best known for their database.
Oracle provides a debian repository at http://oss.oracle.com/debian, containing only Oracle 10g Express Edition (XE) packages. To see these packages, add this line to /etc/apt/sources.list file:
deb http://oss.oracle.com/debian unstable main non-free
Next, you will need to add the GPG key.
wget http://oss.oracle.com/el4/RPM-GPG-KEY-oracle -O- | sudo apt-key add -
As root, type:
To install the XE database server, apt-get 'oracle-xe-universal' or 'oracle-xe' (Western European support only).
apt-get install oracle-xe-universal
If you only need the XE client, type
apt-get install oracle-xe-client
More information about the Express Edition is available at http://www.oracle.com/technology/products/database/xe/index.html
Oracle Instant Client is another option for installing an Oracle client, with more recent versions available.
Oracle11g, feel free to include this howto in this wiki.