Watching SL Full time last night, I'm not sure the shoulder hits him on the head.
I think the contact is shoulder to shoulder, but since the froggy is a bigger guy the wall created by his shoulder goes up higher than O'Briens. When they come together, this means that the point of his shoulder is in the vacinity of O'Brien's head, but I'm unsure of any real contact - I think the concussion could have been caused by the
whiplash effect of being completely clattered when relaxed.
Its still a late tackle, so still illegal, but I think its another bit of evidence that O'Brien needs to learn to look after himself better.