Oviedo is not a good option because the airport is very far from Oviedo, just between Gijon, Aviles and Oviedo but near to Aviles.
In my opinion the easiest way to Astorga is by bus from Madrid (Estacion Sur, South Station).
There's several buses, the non-stop takes about three hours, the others a bit less than four hours (stopping in La Bañeza, 23 km from Astorga).
Buen Camino,
Javier Martin
Madrid, Spain.