What you guys are used to monitor in grid infrastructure clusterware?
I would monitor
Oracle High Availability Services : ohasd process
Cluster Ready Services : ocrd process
Cluster Synchronization Services : ossd process
I use Oracle's Enterprise Manager 12c Cloud Control. When you install the agent on the nodes, it is cluster-aware and even treats the cluster as a single managed target. It can send alerts when a piece of the cluster is down.
I researched and there are OSwatch and Cluster h monito.
You can certainly use a utility like OSWatcher. But it won't alert you to anything. It will just write to log files.