How do I enable READ-only (passive) mode in a particular RDBMS?
I'm going to attempt to answer this question for Oracle.

Historically Oracle has never supported read-only databases.

It's possible to make a tablespace read-only: http://docs.oracle.com/cd/B28359_01/server.111/b28310/tspaces006.htm

Individual tables can be made read-only: http://www.oracle-base.com/articles/11g/read-only-tables-11gr1.php

Or it's possible to make a select-only role and assign that to the user who logs in rather than the normal readwrite-role.