superesse

wp-search-inorcondition allows you easily to set the “OR” condition for “every single word” of your research “s” querystring value
Released under the terms of the GNU GPL, version 2. 
http://www.fsf.org/licensing/licenses/gpl.html
NO WARRANTY.
Copyright (c) 2011 solamentegratis.it
<h3>Arbitrary section</h3>

Search in or condition

Adjust your conditional "OR" operator on the wp search query

felthy

This is a very simple WordPress plugin that makes the WordPress search box find posts that contain ANY of the words 
entered instead of only posts that contain ALL of the words; that is, it makes the search use the OR operator instead of 
the default AND operator.
It doesn’t have any settings or configuration – just place the plugin in your <code>/wp-content/plugins/</code> folder, activate 
the plugin through the ‘Plugins’ menu, and searches that have more than one word will start returning posts that contain 
one (or more) of the words instead of just posts that contain every one of the words.
Easy.

Any Word Search

Makes the WordPress search box find posts that contain ANY of the words entered.

Search in or condition vs Any Word Search