Dont worry too much about the Maths. If you got a C in GCSE then you meet the entry criteria. However you are going to have to work hard at Speed Distance Time calculations for OASC. I have coached a couple of guys through selection who thought they had no chance with maths.
You can make yourself a much stronger candidate than a mathematician, if you crack the mental arithmatic and SDT calcs. There are lots of books out there that will help and worth a look. But the only real solution is to work your ass off. What I recommend is to make yourself a set of questions, say 100 per page. Include addition, subtraction, division and multiplication. Also include SDT questions so that you can instinctivley solve them. Concentrate on speeds and times that are not too complex such as 3mph, 6mph, 12mph 20, 30, 60, 120, 240 etc You will be given an exercise that involves extensive calculations and will be expected to think on your feet. "If the boat was travelling at 12mph for 20 minutes how far would it travel? how about at 20mph for 6 minutes? Or 30mph for 15mins?" What speed was the aircraft travelling if it took 24 minutes to cover 120miles? Practice, Practice, Practice. Use all the quiet moments, Keep a set of questions on you, put some in the toilet. The effort will be worth it. If you can get the sums instincive you can go to OASC with a lot more confidence.
You should also understand basic vectors and Trig. Keep it simple and you wont stress yourself out.
Good luck