Have you had a good look at their website? (I just have and found it quite confusing, despite being a past OU student and thinking I knew how their set-up works!)
I started studying with the OU when I was in my mid-twenties, having left school with 1 A level, so don't let lack of A levels put you off.
The 'Openings' (ie introductory) courses are here:
www.open.ac.uk/study/explained/study-explained/our-range-courses/openings-courses
They're short courses at level 1 and are worth 15 credits towards a degree (360 needed in total). The standard, year-long modules are worth 60 credits so you would need to do six of those to get a degree. They also do some shorter modules with 30 credits.
You used to be able to mix and match modules, so you could have studied both business and psychology modules. I don't know if you can still do that as they seem to have standard 'pathways' to a degree, which is more in keeping with how traditional universities work.
Actually I've just found the following, so yes, if there isn't a 'named' degree that you like then you could combine business and psychology modules into an 'Open' degree:
"For a degree in a named subject, such as business studies or humanities, you'll study mainly in your chosen subject, although you may be able to include some modules from other subjects.
With The Open University you could also study an Open degree where you can combine a range of modules from similar or different subjects to suit either your particular career or personal interests."
It may be better to start on this page where you can view all the modules by subject and see which ones really interest you:
www3.open.ac.uk/study/undergraduate/course/index.htm
Then work out whether they offer a standard 'named' degree that suits you or whether you'd prefer to mix and match an 'Open' degree.
Start by looking at social sciences as that includes psychology.