In the OG API, which hooks are available when a group admin adds or removes a user from a group?
You can find all OG hooks here.

You already mentioned two of them, but for the sake of a complete answer here are the hooks fired on membership add/edit/delete:


hook_og_membership_insert
hook_og_membership_presave
hook_og_membership_delete


Also, as memberships actually are entities you can also use all hooks used for saving and deleting entities, like hook_entity_update and hook_entity_delete. You can find them in this list.