Hello again,
You do need credentials to discover ESX. ESX 4.x uses SNMP ESX 5.x uses WBEM. SNMP needs a community string like public and to be allowed to connect to the SNMP deamon running on the ESX server. ESX 5.x needs root or a user with root rights on the server. I would recommend to create a SIM discovery user with root rights on the server. No extra credentials are needed on OA's or ILO's for discovery. The ILO responds on http and snmp by default.
The OA and ILO use a xml page build in in the web interface, no extra credentials are needed for discovery. However if you want to receive events you need to setup the SNMP trap settings on the OA and ILO's.