OID which stands for Oracle Internet Directory is part of Identity Management in Infrastructure Tier of Fusion Middleware. If you are planning to configure 10g Application Server with Apps 11i or R12 , This note might be useful for you in understanding OID . Looking at importance of OID, I am going to discuss on few important things about OID today .
OID is part of infrastructure tier in 10g Application Server (Identity Mangement from 10.1.4 onwards)

What is OID ?
Oracle Internet Directory (OID) is Oracle’s Implementation of LDAP (Light weight Directory Access Protocol) which is ldap version 3 compliant. OID is special kind of database repository in which information is stored in Tree structure also called DIT (Directory Information Tree).
Similar to OID , Microsoft has its own LDAP server called Active Directory (AD) and Sun’s LDAP server is called as iPlanet .

Where is OID code in oracle_home ?
OID code & its corresponding log files are stored in directories under $ORACLE_HOME/ldap directory in Infrastructure Tier . This is same tier where your SSO server sits.
OID logs are stored at $ORACLE_HOME/ldap/log (This location is quite important for apps dba’s for troubleshooting OID Issues). Few executables like oidctl, oidadmin, oidca, oidldapd are in ORACLE_HOME/bin .

What are default ports for OID ?
You may see different ports for OID depending on OID server version but most common is 389 non ssl OID port & 636 for SSL OID port (These are also default ldap server ports). If you don’t know which ports your OID is using refer portlist.ini in $ORACLE_HOME/install (Note that this file will not list updated port if you change OID ports after Installation)
You should an entry like
Oracle Internet Directory port = 389
Oracle Internet Directory (SSL) port = 636

How to start OID ?
OID process is controlled by opmn (Oracle process monitor & notification server) so you by default use opmnctl command
To Start OID opmnctl startproc ias-component=OID
To Stop OID opmnctl stopproc ias-component=OID

OID can also be started without OPMN by
First start oidmon (OID Monitor Process)
Then use oidctl (OID control)
To stop OID without OPMN
First stop oid process using oidctl then stop oidmon (OID monitoring process)

When you start services using opmnctl , it inturn start oidmon & oidctl .

How to troubleshoot OID issues ??
Where to check for OID Logs ??
What is OID Replication Server ??
What is Integration & Provisioning Server in OID ??
Coming soon …..