Dear Sourav, Both Oxford and Harvard are one of the top universities in the world. The admission process is for both happens to be very competitive. Oxford receives approximately 19,000 applications for 3250 seats each year. The entry requirements for Indian students apart from others are: Year XII qualification, studied with either the CBSE or ISC examination boards, with an achievement of 90% in each of the five subjects studied. Science subjects may also require that candidates take the Indian Institute of Technology (IIT) entrance examinations as a pre-condition for entry. Harvard requires a Mandatory SAT/ ACT score for admission to undergraduate programs. YOU will have to apply through the Common Application, Universal College Application, or Coalition Application. The admission process is very competitive. Summing it up, if you are good in your studies, extracurriculars, application essays, its easy at both places and if you are not...it would be tough. What I would suggest you is that you decide between CSE and Maths first.

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All the best!