"Finally, the fatal flaw in the "Obama didn't go to Columbia" theory is that he couldn't have been admitted to Harvard Law School in 1988 without having received an undergraduate degree. If he wasn't attending Columbia from 1981-83, he would have had to complete two full years' worth of coursework at (and graduate from) some other accredited college — yet his time between the end of his Columbia days in 1983 and his entering Harvard Law in 1988 is accounted for (working at the Business International Corporation and the New York Public Interest Research Group, then serving as director of the Developing Communities Project in Chicago), and no other school claims him as an alumnus, nor does anyone purport to have encountered him as a classmate or student at any other college or university during that period."
http://www.snopes.com/politics/obama/columbia.asp