There are two ways to search with boolean operators in ONESearch. First, you can type them directly into the ONESearch's Basic Search box. AND, OR, & NOT, must be typed in all UPPER CASE to register as boolean operators.
The second way to use boolean in ONESearch is to select them from the drop-down menu in an Advanced Search. First, click on the "Advanced Search" option at the top of the results page, next to the search bar: