OG group content notification for group administrators
Your task cannot be done in an obvious way. A piece of content can be content of several groups. If a user is admin in the first group, but not in the second group, should he receive the email or not? If a user is an admin in both groups, should he receive the email twice?
You'll have to think about that first before one can give a precise answer.

Either way, my guess is that one should create either a rule component "Send email if group admin" that receives a user and a group as variables, or one should create a view of group admins that the rule loops over. Do ask me for details if you still need the advice.