Hi James, Welcome to the OU thread and to TSR!
I am starting my Social Sciences degree with the OU in October and will, like you, be aiming to complete it in 3 years, 4 yrs max.
Lots more information can be found on the OU website; but from what I know 120pts is the maximum you can do in a year (and thats ok- because I will be running two 60pt courses from October the 6th for 9months) though I have to say, they will try and stop you, and phone you up to check you know what you're letting yourself in for- this is very much dependent on what previous study you have done, and what level the courses are (although there are no entry requirements- going into study after A levels/FE or even HE is very different from having not studied for 25 years after leaving school at 15). Phew! Sorry about that waffle!
Anyway, on top of the 120pts you are allowed to do a (or prob more than one but I'd say 1) stand alone residential school (i.e. not a course which has a residential school, but just a purely residential course which will prob be 15 pts; so you might not think of counting this towards your 300/360 total as you want to do it so quickly, but you could look on it as an experience!)
Hope that makes some sense,