It'll depend on the modules that make up the BSc and how many credits they are worth. The course I studied started off as 30 credit modules so could potentially do 3 but in later years the minimum was 60 credits. Have a look on the website at the specific degree route
Does "flexible study" mean I could technically complete the degree in less than 6 years? And get more than 60 credits in a single year?
I can't do 36 hours a week (full time) but I can certainly do more than 16 (part time).
Yes, you definitely could - quite a few people study OU full time, and complete their degree in 3 years. When you first apply, someone will probably contact you to check that you're not taking on too much.