Oshkosh Panhellenic
This is the UW-Oshkosh Panhell site. GAO is not a member of the local panhell, so you can probably join an NPC group as long as your GAO chapter does not prohibit it.
However, it does look like GAO is considering joining Panhell, so then you would probably not be able to. You could ask your Greek advisor or the Panhell president for more info.