Enable Oracle Database options in an existing database I want to enable Oracle Real Application Testing option in my existing database, but I couldn't find the way to do that using Oracle Universal Installer (OUI) or Database Configuration Assistant (DBCA). Do I have to reinstall the Oracle software in order to enable the options that I didn't choose to install during Oracle software installation?

The version of the Oracle Database installed is 11.2.0.3.
No, you don't have to reinstall the Oracle software. According to the section "Oracle Database 11g Release 2 (11.2.0.1) New Features" in the Database Installation Guide 11g Release 2:


  Oracle Universal Installer no longer provides the custom installation
  option of individual components. Use the chopt tool, a command-line
  utility that is located in the ORACLE_HOME/bin directory, to configure
  the database options.


For example, to enable Oracle Real Application Testing, you would first shut down your database, and then issue chopt as follows:

chopt enable rat


For more information on how to enable and disable other database options with chopt, consult "Enabling and Disabling Database Options" section in the Database Installation Guide 11g Release 2.