What are the transportation options between Porto and Santiago de Compostela? Are there any options other than renting a car?
We took a bus from Santiago to Porto, so I suppose it goes the other way, too! I would suggest that you buy your ticket in advance, maybe the day before.
Find the Portuguese schedules at https://www.cp.pt/passageiros/en/, then the Spanish portion of the trip through Renfe or the usual DB schedule search.