Great question is asked on EPT blog:
What would be the consequence of just a single butterfly wing-flap in, say, Sweden on some new medical development in Peru?
For researchers to be able to use a new development from the opposite side of the world, such development simply has to be be available. Without OA, knowledge cannot have the same impact as it could have if it’s freely available soon after it’s created. But OA is one part of equation – the second is to create system in which relevant knowledge is findable. I’m not sure if butterfly effect is very frequent when the butterfly does not have possibility to fully interact with the whole system.