Oracle Enterprise Manager 11g (11.1.0.1.0) agent installation using response file

Download and extract agent's binaries in a directory in your server, e.g. /var/opt/oragrid/binaries.
Open file /var/opt/oragrid/binaries/hpi/response/additional_agent.rsp and do the following changes:

SECURITY_UPDATES_VIA_MYORACLESUPPORT=FALSE
DECLINE_SECURITY_UPDATES=TRUE
#MYORACLESUPPORT_USERNAME=
#MYORACLESUPPORT_PASSWORD=
#COLLECTOR_SUPPORTHUB_URL=
ORACLE_AGENT_HOME_LOCATION=[directory where agent software will be installed, e.g. /var/opt/oragrid]
OMS_HOST=[server's name where OMS is installed]
OMS_PORT=[server's port, default 4889]
AGENT_REGISTRATION_PASSWORD=[sysman's password]

If you are installing an agent in a RAC, change also:

CLUSTER_NODES="[node1's name],[node2's name],..."

If you don't want to mess with server's oracle central inventory, use a custom inventory, by creating an oraInst.loc file, e.g. /var/opt/oragrid/oraInst.loc.
Open it and add:

inventory_loc=[path where this inventory will be placed, e.g. /var/opt/oragrid/oraInventory]
inst_group=[agent's user group, dba or oinstall]

Go to /var/opt/oragrid/binaries/hpi/agent/install and run:

runInstaller -invPtrLoc /var/opt/oragrid/oraInst.loc -silent -responseFile /var/opt/oragrid/binaries/hpi/response/additional_agent.rsp

If you are installing in a RAC, find cluster name:

cd $CRS_HOME/bin
./cemutlo -n

Now, run:

runInstaller -invPtrLoc /var/opt/oragrid/oraInst.loc -silent -responseFile /var/opt/oragrid/binaries/hpi/response/additional_agent.rsp -CLUSTER_NAME=[cluster name] CLUSTER_NODES="{[node1's name],[node2's name],...}"

There is a bug that fills emagent.trc pretty fast.
So, at the end, install patch 9756966.

Finally, run [agent's home]/root.sh as root.

In the RAC installation, all onoff patches that were supposed to be installed after agent's installation, failed, because although I had specified my custom oraInst.loc in the runInstaller command, OPatch was using oraInst.loc under agent's home directory.
Probably, you will not face this issue, if you don't specify a custom inventory.
If that happens also to you, go to [agent's home]/install/oneoffs/111010 and install them manually.

If you try to monitor a 9i database not in 9.2.0.8.0 patchset, you will probably get many ORA-07445 in the alert log and core dumps in your database's cdump directory.
To fix this, edit [agent's home]/sysman/config/emd.properties and add the parameter AvoidOCIPing=1.
Then, restart the agent.