Hi, David,
Actually, there is clear marking if you want to get to Oviedo from the Camino del Norte. The turn comes after Villaviciosa, and from there to Oviedo is about 45 km. It's all very well marked.
As far as albergues go, in fact there are some. One requires a detour (again well marked) to Valdedios, which you will see a few km after you get on the camino towards Oviedo. I don't have the exact number of kms, but I left Villaviciosa early in the morning and was at the church in Valdedios around 11. I highly recommend this detour, it's an unbelievable pre-romanesque church in a meadow right next to a 16th or 17th century monastery, and there's an albergue there as well.
When I walked, I spent one night in a hotel in Villaviciosa, then walked to the church/monastery at Valdedios, and then spent the night in a hotel in POla de Siero (Siera?) which is about 16 km outside Oviedo. But there is a town, about 8 km before Pola de Siero, which has a very basic albergue. We decided to keep going. I'm sorry, but I can't remember the name of the town now, but it is definitely a do-able day's walk from Villaviciosa. I hope this makes sense. But if you have any questions, let me know. Buen camino, Laurie