These two templates should assist with monitoring OpenSearch clusters with Zabbix. I recommend using both templates, as they monitor different things.
The OpenSearch template monitors individual nodes. Triggers are focused around issues with the specific node such as it being offline, or performance issues on the node. The OpenSearch Cluster template monitors the cluster as a whole. It discovers all indices, and has triggers if there are issues with those indices, or around the whole cluster's status.
See the each templates README.md file for assistance on configuring them.
This template only uses HTTP Agent requests and the API calls will come from your Zabbix Server or proxy to your OpenSearch node. Ensure all relevant firewall rules are in place to allow that traffic.
Please reach out with any issues you discover, or things you'd like to see added through the repo's Issues.