About 100,000 people will be taking the exam May 10, and a large number of them will earn 4s and 5s. If you stay calm and focused, there's every reason to expect that you'll be one of them. They never ask an open essay question that applies to a handful of works, so if you know and understand HoD, you will probably be able to base an essay on it. They will be worded in such a way that you can argue any number of positions. The first two are pretty straightforward.
Opposition refers to pairs of elements that contrast sharply.
The obvious oppositional pair in HoD is black and white, and you can find a great deal written about it in the postings below. In addition, pay attention to oppositions of characters to one another, of characters to settings, of style to subject, and so on. Once you attune yourself to this idea, you can find them everywhere. Your real job is to decide how they support whatever theme you've picked for your essay. The readers are trained to read each essay very rapidly, assign a score on a 1-9 scale, and move on.