Hi there,
I think I can definitely relate to this post as I'm currently just about to start year 4 / 6 of Economics & Mathematical Sciences in October.
Pre OU I'd gotten a C at GCSE maths and D in statistics.
Here's what I did:
before the start date I crammed in as much math revision starting from literally multiplication and division all the way through to pre calculus stuff. I had a friend teaching me occasionally as well as khan academy and a couple a level textbooks for about 3 months.
1st Year: This got me kind of up to speed but still felt quite under prepared. I did 3 modules the first year (90 credits) which were MU123, MST121 and DB123 , I passed MST 121 and DB123 although it was a bit of a scrape for MST121, I think that the only real factor stopping me from doing better was I hadn't found my work ethic yet and whilst I did do work I left most things to the last minute, was always behind on the books and struggling to catch up whilst learning how to maintain a job and study at the same time. I didn't sacrifice enough social time to get a decent grade - but at least I passed and I knew the marks wouldn't count anyway so... it was what it was. I wouldn't recommend with full time jobs doing more than 60 credits per year, 90 was way too much to handle so bear that in mind - I made the decision at this point to only do 60 per year (and the 60 credit modules feel like a lot less work than 2 x 30 so keep that in mind).
2nd Year: Next I did M140 stats and DD131 social sciences, the stats was really interesting but again this year I was better but still hadn't quite got the work/study/life balance yet, so I did better this year but not fantastic. Again marks didn't count so not too much of a problem. The stats module didn't really require any prep - just raid all the stats videos on khan academy would be a good enough preparation and just make sure to keep ahead of the books because there was a lot to read.
3rd Year I did DD209 - Economics 60 credit module - this was a great module, really enjoyed it and managed to get my head around good essay writing and even with one assignment rushed due to other things going on in life at that particular time managed to get a distinction overall so very pleased. only requirements for this I'd say would be understanding lots of types of graphs and charts and how changing variables will affect them.
4th year - So i'm approaching MST224 math and M248 analysing data - I've received my books already and there is some really cool but hard looking maths. I've been working on the bridge material that OU supplied for MST121 -> MST224 to get myself up to scratch having not done math in 2 years and also not having done especially well even then. It's mostly calculus and complex numbers that you'll need to be comfortable with as a prerequisite. So now I've found my work ethic last year I know how much needs to be done in order to succeed. Every single day working / weekend or whatever must have at least some studying done - I prefer lots of short bursts rather than ten hour slogs as I feel I learn and retain faster. I also have a friend who will occasionally teach me which is very valuable - paying for extra tuition wouldn't be a bad idea if you can find someone good who will teach at the appropriate level. Hopefully this year will be the turning point for me and maths where I can get myself to a really good standard by the end of it.
Overall the main things I'd bear in mind is that:
Mathematics is cumulative - not everyone who is good at maths is a genius, most have built up lots of knowledge over time and many hours of work and study so don't feel bad for not understanding things straight away, your work ethic is really what will make or break your results. Get the work / study balance right and your grades will improve for sure.
Use all resources - Mathematics is universal so if the book isn't coming at you at the right angle (Sorry I couldn't resist that one..) then find websites, videos, friends that can show you from a slightly different viewpoint and you'll start to connect the dots.
Good luck with your course!
Simon