I did OCR History too and came away with an A. In AS we studied the American Civil War (pretty sure the exact module you listed above) and The Wars of the Roses, in A2 for the Historical Theme module we did The Ascendancy of France and the Historical Investigations module was Napoleon (again I think pretty much the same module you listed above but at A2 the dates were 1799-1815).
If you're doing history you will have to work hard, from this I mean don't put off until the last minute, there is just so much information to take in that cramming is just not a desirable route to take.
As Morgan141 said, lots of essay practice, redo the same ones, write out essay plans, also get into the habit of doing the essays in the time you will have during the exam i.e. 45 minutes (admittedly to begin with it will take you longer).
When I was in year 12 my history teacher set us the challenge (we didn't have to do it) of doing 14 essays over the Easter holidays (a tradition created when a student from a while back came in after the holidays with 14 essays), I know it worked out as 1 a day but I ended up doing a few all in one day, doing the 14 essays was the best thing I could have done, and I did a similar thing for the A2 exams.
Comparing my two years I did better during AS than A2, and I would advise doing as well as possible in Year 12 because it takes a lot of the pressure off knowing that you have the option to screw the final exams up a bit (thankfully I didn't :P).
...hmm what else... Read around the subject as much as possible and try not to explode with frustration during year 13 - the historical theme module has to cover a minimum of 100 years and I suppose the whole point of the module is that you understand the themes, ('change' and 'continuity' were the buzzwords on the OCR website -that reminds me coming up to the exam read the examiner's reports on your modules, I found it very helpful seeing where, in general, other candidates fell down or did well) but the amount of information you are given makes you cry with "I'm not going to remember all this!" but don't panic you don't have to remember every little nuance to the extent you do in the AS's, oh that also reminds me...
...don't try and write every single thing you have learnt during the year into the essay, keep to the point answer the question written on the paper not what you want the question to be.
Then I suppose, most importantly, enjoy the course! Hope that helps.
p.s. sorry for the frequent digressions...