The arithmetic questions vary considerably in difficulty. The first ones start off along the lines of....
Your aircraft is travelling 150mph, how long to travel 25 miles?
They then become more complicated such as...
Aircraft A leaves station A at 1100hrs travelling 350mph. 10 minutes later Aircraft B leaves station B travelling at 285mph. The stations are 600miles apart. At what time and how far from station B will the aircraft meet. (or something to that effect).
At the end of the day you are still doing no more than basic DST questions with a few complications thrown in. From what I remember this was about the only specific arithmetic exercise (though I am sure someone will correct me if I'm wrong) so just practise a bunch load of DST questions and you should be home and hosed. As with most of the tests you will probably be thinking how hard it all seems and how much you are being fried, but relax - the tests are written to accomodate for the most intelligent individuals so you are not expected to get through them all.
Topgun, most of the aptitude tests are individual, however there is a specific test which measures ones ability to multi task. It does this by getting you to remember a sequence of letters whilst doing basic arithmetic (i.e 97+25=) and getting you to cancel out shapes that fly across the screen. From my experience (and I have had the pleasure of doing 3 lots of aptitude tests in 2 countries) I believe one cannot do a lot to enhance their scores at OASC. However practice some arithmetic and sharpen up your hand eye coordination on flight sim or the likes as this will greatly increase your confidence - if there is one quality that potential candidates lack most it is confidence.
Once again, best of luck to all heading up to OASC.
(ps. my spelling is not normally this bad, honest!)