Thank you very much for your prompt reply, William 512. I have 2 further questions:
(1) Is it required of us to reserve the bus tickets in advance?
(2) Looking at the walking course map of Oirase stream, I should be sitting on the left side of the bus (coming from Aomori) as this faces the stream?
When we took the bus during Feb (winter service nowadays is suspended) we booked our seats beforehand but the bus was less than half full on the day of travel. By autumn foliage season you can expect a lot more visitors, so it would be safer if you book your seat in advance.
Going towards Towada-ko you should sit on the right-hand side for view of Oirase Stream.
Thanks again for your help, William512.