The Friends of the Osterhout Free Library was formed shortly after the Agnes Flood of 1972. The group began by raising funds to help the library through the sale of bookmarks, cookies, and books, as well as hosting wine and cheese events and fundraising dinners.

Eventually, the annual Book Sale became the main fundraising event coordinated by the Friends. Throughout the year, members sort and organize books which are donated by library patrons. The third week of June is when the annual Book Sale is held on the library lawn. In 2007, the Friends raised $23,000 by selling books. Their goal each year is to surpass the previous year's profit. All money raised by the Friends goes directly to supporting the library's mission.