http://portal.acm.org/citation.cfm?id=1940819

This study takes 12,354 original research articles which were published in 93 Oxford Open journals in 2009 as a sample, and carries out statistic analyses on the citation frequency that these articles have received by July 2010 to validate 3 hypotheses: (1) there is citation advantage for open access articles(OACA) published in Oxford Open journals over the non-OA ones; (2) OACA varies with disciplines; (3) there is some correlation between the impact factors(IFs) of Oxford Open journals and the OACA of their open access articles. This study discovers that: there exists OACA for open access articles, in this case 138.87% higher over non-OA ones; different subjects have different OACAs, and Humanities journals in Oxford Open have even a negative OACA; Oxford Open journals with lower IFs have stronger OACAs than those with higher IFs.

Update:
http://www.istl.org/10-winter/article2.html
"Studies typically show a 25-250% OACA or more"