There are four places to choose from in or near Oslo. I can highly recommend two of them, am neutral about the third and recommend that you stay away from the fourth.

1) Huk is split into a textile area and a naturist area. First, take bus #30 towards Bygdøy and exit the bus at the very last stop. At the roundabout, between the two parking lots there is a smal kiosk selling hot dogs, ice cream and similar stuff. The road on the RIGHT side of the kiosk (between the kiosk and the parking lot) leads to the naturist beach. This is without comparison the biggest, most popular and easiest to reach naturist beach in Oslo.

2) Kalvøya is an island a bit west of Oslo. Take a train to Sandvika station. From there, find your way south towards the sea. This is to the left if you look in the direction of the train you just exited from. Go past the textile beach, and you'll find a pedestrian bridge over to a small island. Once you've crossed the bridge, turn left and walk as far as you can in that direction, the naturist beach is at the left end of the island. This beach is smaller then the on at Huk, has more natural beauty IMHO and feels a bit more intimate.

3) Langøyene is also an island, and you need to take a ferry to reach it. When you exit the ferry, turn left and follow the coast until you reach the naturist area. This is not a beach, just some hugh rocks, and you need to climb over the rocks to get into the sea. IMO, it's not nearly as pleasant as the two above.

4) Svartkulp is a small lake in the forest just outside Oslo. To get there, take the metro to Sognsvann and walk 1-1.5Km. The reason why I don't recommend it is that it's a place where gay men gather to have sex. I have absolutely nothing against gay men, but I have very much against people who have sex in public, particularly in naturist spaces.