Can I get into university or college by just doing grade 11 and 12 of US curriculum high school?

I have been doing Grade 9 and 10 from Edexcel 'O' Levels (British). And now I have to move to an American school for Grade 11 and 1. Can I get into a reputable college or university e.g. Preston, Harvard etc? What about credits?Can I acquire the necessary credits in Grades 11 ad 12?