Sent email to UN on this and this is what I got in return:
Greetings from the UN Reference Team.
The Organization for Security and Cooperation in Europe is an [i]nternational organization[] having received a standing
invitation to participate as observers in the sessions and the work of the General Assembly and maintaining permanent
offices at Headquarters". http://www.un.org/en/members/intergovorg.shtml.
wHAT DO YOU ALL THINk?