Santander to Estella/Santo Domingo to santander

Grainne,
Since I don't know where you left off last time, I'm not sure how to help. But I flew from Dublin to Santander some years ago and had a beautiful 3 hour bus ride down to Burgos. The Santander airport is so close to the centre of the city and the bus station that it worked out very conveniently. Presumably there would be a similar bus route to Osorno (or possibly Frómista on the Camino F). You can check bus routes quite easily on www.movelia.es (in English).

I would take the local bus (leaves every 3o minutes) or a taxi from the Santander airport to the bus terminal, and then ask for the more convenient ride. Not everything can be planned in advance (but that´s me...).
Both Estella and Santo Domingo are out of the way cities. You will need probably plan a trip to the nearer well connected city, then take a local bus to your destination. Assuming you are beginning in Estella, ALSA , for ex., http://www.alsa.es/ has some services Santander - Logroño, and it is even possible (depending on your flight schedules) to board in the airport (select Aeropuerto Santander). Then, in Logroño, Estellesa (there are other companies) http://www.laestellesa.com/ to Estella.
Buen camino!

Apologies--I really must learn to read more carefully! Of course you said where you wanted to go and return from. Felipe gives much better advice!

ALSA connects Santander Airport to Logrono and Burgos

Estellesa has a route between Logrono and Pamplona with stop in Estellesa

Autobuses Jiminez has a route between Logrono and Burgos with stop at Santo Domingo de la Calzada

Santander Airport
http://www.aena-aeropuertos.es/csee/Satellite/Aeropuerto-Santander/es/

ALSA
http://www.alsa.es/en/

Autobuses Jiminez
http://www.autobusesjimenez.com/

Estellesa
http://www.laestellesa.com/

Logrono
http://www.horario-autobuses.com/horarios-autobuses/la-rioja/logrono.html

Burgos
http://www.aytoburgos.es/turismo-en-burgos/como-llegar/estacion-de-autobuses-de-burgos