Oracle Enterprise Manager

Configuring Oracle Enterprise Manager for the first time

export ORACLE_SID = prod

sqlplus / as sysdba

select username, account_status from dba_users order by 1;

From the listed users we need SYSMAN and DBSNMP users for OEM functioning.

select username, account_status from dba_users 

where username in('SYSMAN','DBSNMP');

Both accounts are locked.

To unlock them and reset their password:
alter user sysman identified by manager account unlock;
alter user dbsnmp identified by manager account unlock;

configure a listener to use with OEM:

netmgr
listener name list_prod
port 1530
lsnrctl start list_prod

$emca -config dbcontrol db -repose create
database SID: prod
listener port number: 1530
listener home:
pwd for sys user:
pwd for dbsnmp user:
pwd for sysman user:
yes

once the configuration completes successfully 
note down the database control URL
Open it in firefox browser in linux.

this all for the first time configuration.  once done to use OEM we need the following:
1. database must be started
2. listener must be started
3. emctl service must be started

sqlplus / as sysdba
startup;

lsnrctl start list_prod 

emctl start dbconsole

to start oem in chrome or firefox
https://147.43.0.13:1158/em/

here 1158 port number is for prod database. we get a diff port number for each of the databases which we configure with OEM.