I received the same request once .. and what I need is this module og_role_override
This module provides Core permissions to act as particular OG roles in
specific group types.
In other words, it creates permissions of the form 'Act as ROLE in
GROUP-TYPE' for each role and each OG group type. Granting this
permission to a Core role gives it the same permissions as that OG
role, in all groups of that type.
For example, you might want users with your 'site editor' role to be
able to act as a 'board member' in all OG groups, without having to
add them to each group, and without having to replicate all the OG
permissions as Core permissions.
Hope it helps ..