Welcome to Oslo and hope you have a nice stay aviationluver!
It's generally a nice time of the year to visit, you'll just have to cope with the moody weather
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In regard to your question on converting your FAA to JAA, you might find useful information at the flight school "Luftfartsskolen" who have their main office in Wergelandsveien 3(street adress) downtown Oslo. It's a fairly long process, especially the theory. Nevertheless, follow your dreams!
For an aerial tour operator you could check out "Kjeller" airport in Lillestrøm which seems to have most of the GA movement in the area.
Best regards,