The Oxford academic year commences in October and finishes in June each year. The academic calendar consists of three terms (Michaelmas, Hilary and Trinity).
You can only take courses offered by the Engineering Science Department and not from other Oxford departments. It is expected that B-papers (third year courses) and C-papers (fourth year courses) to make up the majority of the courses to be taken by NUS students. The normal workload of an exchange student at Oxford is a total of 6 Examination Units (EUs) as written papers plus a project with a weighting of 6 EUs. You are expected to take no more than four B- and C-papers plus one A-paper or six B- and/or C-papers. Each A-paper (second year course) is twice the weight of a B- or C-paper, i.e. 2 EUs. One thing to note, taking of A-papers are subject to the necessary agreement from the tutors at the colleges involved in the exchange.
Engineering Science Handbook will give you an overview on the course curriculum offered by the Dept of Engineering Science at Oxford U.