(a) A General OA Advantage: the advantage that comes from citable articles becoming available to audiences that had not had access to them before, and who would find them citable
(b) An Early Advantage: the earlier an article is put before its worldwide potential audience may affect subsequent citation patterns
(c) A Selection Bias: authors make their better articles Open Access more readily than their poorer articles
(d) A Quality Advantage: better articles gain more from the General OA Advantage because they are by definition more citable than poorer articles
The really nifty chart that is part of this article summarizes 31 published studies in terms of discipline area, study sample, analytic approach, and the assessment of a proven advantage or not.
Result: 27 studies support the hypothesis of bounce
4 studies do not support or contradict hypothesis