OA have a lounge in Schengen and extra schengen. You can only access each with a valid boarding pass for that section.
If you are flying A3 schengen you will only be allowed into the A3 one and not the OA schengen one. Due to A3 not having a lounge in extra schengen you can access the OA one.
I personally prefer the OA one as I hate the smoking room in the A3 one and I have been in there when the door was either left open by the cleaning staff or guests. Sorry I'm big into smoke free zones.Other than that I like the A3 one and it offers similar catering to the OA one.