If you are able to login to oracle as sys then you can be sure that the oracle had started.

To be able to login to oracle as system, if you are trying to use the oracle's system default password, then it would be "manager" and not "change_on_install". You can change the system user's password as

SVRMGR> connect internal

SVRMGR> alter user system identified by your_password;

exit. Now

sqlplus system/your_password@service_name

would connect you in.

To set up the OEM repository, you choose oracle enterprise manager configuration assistant. Before starting with that, make sure that all your tnsnames.ora files have been configured properly. Use "sys" as the username to "logon to the database as a user with DBA privileges."

Sam