Enter OG audience to a node with Rules
This is not the correct answer, but i found a different way. If the og_group_ref is set to checkboxes (these checkboxes are essential) (look at admin/structure/types/manage/[YOURCONTENTTYPE]/fields/og_group_ref), you can set the default audience. 
Now i just hide the field 'audience' for certain roles and new nodes are saved to the right OG.

Again, this isn't the way i asked to do it with rules. When someone knows the right answer how to do it, i'm eager to hear it.