In order to write a successful essay in your ELAT, and thereby allow you to proceed to the interview round in the Oxford application process, it is key to do multiple past papers (these can be found online). The style of the ELAT can be disconcerting, as it is very different from A Level and GCSE English exams: in the ELAT, there is no key way to set out your answer, and you are given a considerable amount of freedom when writing the essay.
I would suggest, based on my expetience with the ELAT, that originality is key with your answers: it is important to find as many interesting points to make as you can, and it is also key that you show a thorough understanding of the set texts. You should be able to approach the essays with a secure knowledge of linguistic and stylistic features of the texts.