If you've forgotten the password for your OMS accounts, then you'll need to reset them. See Metalink note 152597.1 or 100111.1 for details on how to reset the sysman password.
If you have the passwords, then maybe there's another problem. Are you certain the that OMS is up and running and that it hasn't been reporting any errors? Check the log files for OMS to be sure.
This was first published in November 2004