Oracle Advanced Compression—with Oracle Database 11g Enterprise Edition—helps you manage your growing amounts of data (that on average are tripling every couple of years) in a cost effective manner. Oracle Advanced Compression compresses any type of data, including structured and unstructured data such as documents, images, and multimedia, as well as network traffic and data in the process of being backed up. As a result, with up to 3 times less data volume, Oracle Advanced Compression helps you use resources more efficiently and lower storage costs.
For more info, checkout http://www.oracle.com/database/advanced-compression.html
and get some usage tips at http://www.dba-oracle.com/oracle11g/sf_Oracle_11g_Data_Compression_Tips_for_the_DBA.htm