There are multiple daily direct flights from Stockholm Arlanda to Östersund Airport. From the airport to Orrviken is only a twenty minutes drive. Stockholm is reachable from around the globe, making Orrviken reachable with only a single stop from almost anywhere.
Trondheim Airport is reachable via direct flights from a few European cities, for example London, Berlin, Amsterdam and Krakow.
From Trondheim it's a three-hour drive to Orrviken (or two hours with train), through the beautiful Swedish and Norwegian mountains. This option is a better fit if renting a car is an option and you'd want to pair a visit to Orrviken with the beautiful Norwegian fjords and the Swedish mountains.
There are trains from Stockholm, the trip is about 6 hours from Stockholm and 2 hours from Trondheim. On your journey you will see the splendid nature of Sweden.