This document discusses using ONIX, a metadata standard, to describe open access books in the book supply chain. It outlines some key issues for describing open access monographs in ONIX, such as identifying funders as publishers rather than authors and including links to open access licenses. Some proposed additions to ONIX are presented, such as new codes to flag items as unpriced or to link related print and electronic versions, which could be ratified for a future release. Overall the document examines how small changes to ONIX could better support describing open access books in the book supply chain.