See http://www.crossref.org/citedby/index.html

Add Crossref's "Cited by" functionality to OJS, so that we can display any articles that cite a given article in an OJS journal. To do this, we need to add reference deposit functionality to OJS' Crossref XML export plugin -- Crossref only allows cited-by functionality to those who deposit their references. 

Some notes: 

-- Reference deposit to Crossref can happen in two ways: a) during the normal article XML metadata deposit (see http://www.crossref.org/depositSchema/crossref3.0.1.xsd); or b)  separately, as a "reference only" XML deposit (see http://www.crossref.org/depositSchema/doi_citations1.0.1.xsd), which is useful if the article has been deposited previously.

-- Reference deposit more or less requires output from the Citation Markup Assistant. Article reference lists that are extracted/parsed/stored by the CMA then should be a) added to the Crossref XML Export file, which will require an extension of the XML Export plugin; and/or b) available as an XML export option after the Editor has approved all citations, in the CMA itself (along with the current NLM options). 

-- It looks like the XML export information, which up until now has been submitted manually to Crossref by editors, could also be deposited by HTTP POST: see http://doi.crossref.org/doc/SystemInterface.html#asynsubupload. For this to happen, a Crossref login/password credential must be supplied. 

-- Similarly, it looks like "cited by" information can be retrieved by HTTP: see http://doi.crossref.org/doc/RTQueries.html. 

-- "cited by" information should be retrieved, parsed (& added to its own table? how granular?), and displayed with the published article, likely in the RT and/or along with the abstract. I'm not sure how often this information should be retrieved -- every time an article is viewed? 

I'll be investigating this some more and will add notes as relevant. I don't think this will be a trivial task.