Óscar's place is absolutely the best place to choose in Bilbao. The room is quiet, tidy and cosy with a large space. The communication with Oscar/Carlos is smooth and both of them are lovely.
The flat is located at the entrance of the old town, which is very convenient to reach all the pintxos. 3 major tube/train stations are located in 10 minutes walking distance and easy to go anywhere, as well as the taxi point near Casco Viejo station if you are in hurry. Highly recommended!