opHA provides multi-server polling with a central dashboard provided by the Master, it does NOT not handle the realtime events of slaves. Escalations are handled on each Slave server, emails, etc are sent from the slave.
The typical solution here is to have the slave send NMIS events using syslog to the master which are stored in slave_event.log, then opEvents processes this event log instead of the local event.log and opEvents on a single server can handle the events for all slaves. The nodes from the remote slaves will need to be added to opEvents so it knows what to do with them.
With opEvents, any event can be handled from any source, and include escalations, etc. (obviously opEvents running on a Master will carry out all actions as the master not as the slave)