"If you check my reviews over the last couple of years, you'll see a lot of Oslo stuff and Thon hotels in particular. Visiting for work, I've come to trust the brand as decently priced, clean rooms, decent amenities and their price including breakfast.
So, when I and the missus visited Oslo this summer for the Tons of Rock festival, I picked the one Thon branch near the station I'd not yet stayed at. The Astoria. There was a reason for this, as being on Dronningensgate, it was a few meters to the tram stops on Prinsensgate going to Ekerberg, making access to and from the festival very simple. It was also cheaper than the others! :)
Like it's sister branches the Spectrum, Terminus and Opera, it's super close to the Central station (look for the Jernbanetorget exit and head up Karl Johansgate over the road) so there's no need for taxis or other transport and no long walk to lug your bags.
The room was clean, modern and had a decent bed, but was a little on the small side, probably a little too small to be a double. There being an angled wall at one end made getting around the bed tricky with the TV hanging on that same wall! This is probably the smallest room I've had in a Thon so far, so if you want a little more space this might not be the branch for you. We didn't care that much ourselves as we were rarely around other than to sleep.
Staff were excellent as always, very helpful and friendly. Breakfast buffet was decent (as they usually are with Thon) and the missus on her first visit was impressed, especially as it meant she didn't need to give up her healthy fruit and seeds with yoghurt based breakfast for the week! There's also a small patio area out back as you walk to the breakfast/dining area from reception, which was nice and plenty of people made use to eat breakfast in the sun.
If you're flying solo or don't spend 8hrs a day in your room, then go for the Astoria. The location right off Karl Johans is great and there's even two very good restaurants on the same street (Mantra next door and Mamma Pizza over the road, both are top 50 on here for Oslo)."