Hi Vijay,
You likely need two monitors per host in this situation, one for the plug proxy and one for the oracle instance.
If each plug proxy communicates with it's own oracle instance, then you can't simply assign an oracle monitor at the pool level, you have to assign the oracle monitor to the plug server as a pool member.
If all the plug proxies communicate with a single Oracle instance, then the Oracle monitor can be assigned at the pool level
You will need to configure the monitor 'alias address' and 'alias service' settings to point to the Oracle instance on the service port in use (Oracle service ports vary on version, IIRC).
The Oracle monitor can be difficult to configure properly, so you might want to give this a read.
Regards,
Rob