According to Wikipedia (arguably not one of the best sources): A graduate of Columbia University and Harvard Law School, where he served as president of the Harvard Law Review, Obama worked as a community organizer and practiced as a civil rights attorney before serving in the Illinois Senate from 1997 to 2004.
"And keep in mind the timeline here: Obama did this as a young man BEFORE going to law school, becoming a successful lawyer and a law professor."
It's quite clear in the Obama biography on CNN (watched his and McCain's), that he did community organizing AFTER Columbia but before Harvard. He worked for the Catholic Campaign for Human Development.