Oracle Portal is a part of the Oracle Application Server " MIddleTier ".

The MiddleTier depends on the Oracle Application Server Infrastructure ( Metadata Repository ( an Oracle Database ), Oracle Internet Directory ( an LDAP Server ) , Oracle Single Sign On, Oracle Delegated Administrative Services, etc. ). Typically, the Infrastructure is installed first & the MiddleTier next.

Hence, the installer provides various options to install Infrastructure and the MiddleTier Components. The Oracle Application Server 10.1.2.0.2 Documents are here :-

http://download-west.oracle.com/docs/cd/B14099_18/index.htm