Oracle uses an "Oracle Intelligent Agent" to provide SNMP services. Check MetaLink note 69511.1 for the SNMP protocol overview (more references below).
If your only interest in SNMP is to use it from Oracle Enterprise Manager, then you may not need OIA -- the Oracle Enterprise Manager configuration guide can help you.
[NOTE:69511.1] SNMP Protocol Overview
[NOTE:112656.1] How to Verify that the Master_Peer Agent is Working Properly
[NOTE:69625.1] Intelligent Agent and SNMP integration on NT
[NOTE:94313.1] How to Configure the Intelligent Agent with SNMP on Sun Solaris
[NOTE:102796.1] How to Configure the Agent with SNMP on HP
[NOTE:198018.1] How to Configure the Linux SNMP Daemon to Receive EM Event Messages
[NOTE:130790.1] How to Change the SNMP Community Name for the Oracle Intelligent Agent
[NOTE:1065223.6] Error smux socket failure: protocol not supported
[NOTE:76738.1] AIX - How To Configure the Intelligent Agent in an AIX HACMP non-concurrent mode
This was first published in June 2004