Stop oem agent and oms from autostartup on reboot

For OEM agent depending upon the OS below is the location of oratab :

The general EM startup script is /etc/rc.d/init.d/gcstartup.

From the above script you can find the oratab file path for em agent :
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if [ -f /etc/oragchomelist ]
then
     oratab="/etc/oragchomelist"
else
oratab="/var/opt/oracle/oragchomelist"
fi

Just comment the path of agent home and oms home in file /etc/oragchomelist to prevent it from auto-start at server reboot :

[root@grid12c ~]# cat /etc/oragchomelist

#/u01/app/oracle/oms12cr2/oms
#/u01/app/oracle/agent12cr2/core/12.1.0.2.0