OpenSearch
Unfortunately, OpenSearch is two completely different things.
Open Standard
The OpenSearch specification - an open standard for publishing search results came first in 2005. Ironically, it was developed by an Amazon subsidiary. OpenSearch is/was a way for people to directly use search engines from their browser. Famously, MediaWiki includes this capability in the software, which means that you can directly search any wiki site from your browser.
There are many references to this on this site - explaining how to implement and use this search feature.
Software
Almost 20 years later, OpenSearch (software) is a family of software products created by Amazon in 2021 as a fork of Elasticsearch. Now a Linux Foundation project, it is spearheaded by contributions from Amazon, SAP and Uber.
OpenSearch is a Lucene-based search engine that started as a fork of version 7.10.2 of the Elasticsearch service.
OpenSearch Dashboards started as a fork of version 7.10.2 of Elastic's Kibana software, and is also under the Apache License, version 2.
Version 3.0 has been released as of May 6, 2025[1] - based on Lucene v10 and Java 21 plus support for the Java Platform Module System
