Introduction to Oracle for an SQL server DBA

Hello all,
Does any one have any Information for a "SQL server DBA" to know more about Oracle Database from Architecture, Security and other options.
I would like to know the Similarities and differences of Oracle database from SQL Server Point of View. All your help is much appreciated.
With regards
Elias

The concepts manual is probably the best place to start

http://download-west.oracle.com/docs...a96524/toc.htm

Regards