Yes, OSL Lounge is a private lounge, operated by the airport. The lounge is strictly limited to Aeroflot, Air France, British Airways, Continental Airlines, Czech Airlines, Finnair and KLM.

However, until February 1st all Skyteam Elite Plus members were allowed to use the lounge when travelling in coach on Skyteam Flights. I am a Flying Blue Elite Plus flyer and often travelling in coach at Czech Airlines, and I were always welcomed into the lounge. But after February 1st, they decided that only business class passengers and Elite Plus members of the actual airline were allowed to use the lounge: I can use the lounge when travelling in coach on Air France and KLM, but I CAN NOT use the lounge when travelling in coach on f. ex. Continental or Czech Airlines.

So I guess a Northwest Elite Plus card won't get you into the lounge when travelling on any of the Skyteams serving Oslo....unless you get yourself a business class ticket Personally I find this lounge way better than any of Czech Airlines' and Air France's lounges...