Olympiad for MIT or other top-tier schools

The chances of admissions for international students are lower than their US counterparts. Are you one of the top X students in your country? Further I suspect that among the admitted students, there are likely students with major hooks (donor, child of someone famous or powerful) as well as legacy. Give it a shot, but be realistic in your expectations and planning. If studying in the US is your goal, and especially if you are full pay, there are many great institutions in the US where you will be able to land a spot. If financial aid is a must, it will be a much harder road as most schools will be need aware for internationals in the admissions process and many do not guarantee 100% need met even if you are accepted. If that is the case, maybe studying undergrad in your home country and shooting for grad school in the US is a better path as more grad programs will be fully funded.