Östersund is well connected for a city of its size and remoteness. Trains from Stockholm (Centralstation) on the Mittbanan line run several times daily, with the journey taking around 5–6 hours depending on the service — longer but scenic, passing through forests and the Indal valley. The overnight sleeper option is worth considering for the journey: you board in the evening, sleep, and arrive in Östersund in the morning. From Gothenburg, the journey is longer (7–9 hours via Stockholm or Sundsvall). Östersund Centralstation is central and walking distance from most accommodation and attractions.
Östersund Airport (Frösön Airport, on Frösön island) has direct flights from Stockholm Arlanda with SAS and sometimes other carriers, taking around an hour. It's the fastest option if you're coming from Stockholm and don't want the train journey, though the train is genuinely pleasant and the arrival by rail into the station by the lake is a good way to arrive. By road, the E14 runs east to Sundsvall and the E45 runs south to the Swedish interior. Car rental is available at the airport and in the city centre.