Jeff Waugh

Add OpenSearch discovery and querying to your WordPress site, based on the 
<a href="http://www.opensearch.org/Home" rel="nofollow ugc">OpenSearch</a> specification, version 1.1.
After activating the plugin, your WordPress site will expose a standardised 
search interface, accessible to OpenSearch clients such as the <a href="http://www.mozilla.com/firefox/search.html" rel="nofollow ugc">Firefox 
search bar</a>.

OpenSearch

Add OpenSearch discovery and querying to your WordPress site.

Add OpenSearch standard support for search clients to your website implementing 
the <a href="http://www.opensearch.org/Specifications/OpenSearch/1.1" rel="nofollow ugc">OpenSearch Specification 1.1</a>.
This plugin also adds optional support for search engine word suggestions, 
retrieved from the tags used in your website.
The autodiscovery feature embeds a link to the OpenSearch Description (OSD) 
into the head code of your website, allowing the automatic detection for the 
clients that implement the same standard (e.g. Mozilla Firefox search bar).

WP OpenSearch

Add OpenSearch standard support for search clients to your website.

OpenSearch vs WP OpenSearch