Here is the definition of Open eBooks from OverDrive's press release when the new format was announced. These books still mean one copy = one user for check-out. They cannot be returned early like DRM'ed books which has been a recent topic of discussion on MR.
Open eBooks: DRM-free eBooks in both EPUB and PDF formats are being added to OverDrive's catalog for libraries. These OverDrive Open eBooks will be compatible with nearly any eBook reader or software that can display EPUB and PDF files. Titles from Harlequin's Carina Press, Saddleback Educational Publishing, and Rourke Publishing will be among the first DRM-free eBooks available to libraries.