If you are taking examinations this year and you are required to select the options you wish to take, you will be invited by email to enter for your examinations when the examination entry window for your examination opens. The email invitation will ask you to log in to Student Self Service to complete your optional examination entry assessment selections by a given date. Your selections will be validated and confirmed by a series of display screens and you will be able to log back in and change your choices within the examination entry window as many times as you wish.
More information regarding examination entries may be found on the university website, here: https://www.ox.ac.uk/students/academic/exams/entry
The examination timetables on the Chemistry website are the best estimate of the dates at the moment. Please check the university website at the following link: https://www.ox.ac.uk/students/academic/exams/timetables for the only official timetable.