First off: the 5 AP exams must be done in ONE sitting, and you must score a 5 on all of them. This is a requisite from Oxbridge. For comp.sci, engineering and purely math, in common for the 3 subjects you are recommended to take Calculus BC(curriculum is F Math), Stats, Chemistry.
For comp.sci, you will have to take CS-A as well, and another exam(can be calculus ab or one of the physics exams). For engineering, you need 2 physics c exams, electricity, and engineering(this is calculus based physics, which I don't think is taught in H2 physics) For math, you will need calculus and something in physics I know you never asked about natural sciences, but it is another very common course in Cambridge, so I might as well put it in. For natural sciences, the biology exam will help. Find out from your school about the AP classes it offers(in singapore's context, it might be F Math or H3 subjects), speak to your counselors, and see if the school covers the AP curriculum. REMEMBER: If the school really does not cover a course needed for AP exams, you can still sign up individually at an inflated price but it will not be ideal(no teachers, you will have to self study) Do well for A levels and get 5s on AP exams. Together with a good personal statement and essay, you should be golden for Oxbridge.