Elisabetta Poltronieri et al. have published "Open Access Publishing Trend Analysis: Statistics Beyond the Perception" in Information Research.
Here's an excerpt:
The purpose of this analysis was twofold: to track the number of open access journals acquiring impact factor, and to investigate the distribution of subject categories pertaining to these journals. As a case study, journals in which the researchers of the National Institute of Health (Istituto Superiore di Sanità) in Italy have published were surveyed.