Hi! You can either take the bus (search on the website www.alsa.es, there are like 15 buses per day from oviedo) or take the train (http://www.renfe.com/viajeros/feve/).
I think the bus takes less time and it's cheaper (like 5€).
Good luck
I don't think you can go to Cudillero directly from San Sebastian anyway, you will have to stop in oviedo and take another bus.
javier P.
@XMOJICAOtur, Principality of Asturias, Spain
Hi, Alsa (bus) is the better choice from Oviedo.
From San sebatian , (alsa too) ,but the depart it is from Bilbao to Oviedo.
The train is slower but you see more beautiful landscapes.