PKP and Crossref are continuing to collaborate to help publishers and journals using Open Journal Systems take better advantage of Crossref services. PKP is also a Sponsoring Organization for Crossref.
Additionally, this manual covers other Crossref-related services like reference linking, funder information, Similarity Check, and Crossmark. While an official plugin has not been developed for Crossmark, OJS users have been able to use this service with some code modification/addition. It is something that will become available in the future.