I setup an Octopress project following the given instructions (http://octopress.org/docs/setup/) which have you create a Github repository, and create a local repository on
Although people tend to think of their repo on Github as the "official" one, remember that's a social distinction and not a technical one. From git's point of view every repo is on equal terms. That means as long as you have pulled every commit into your local repo, you can safely delete the one on Github. Then just fork the Octopress project on github, set it up as a remote on your local repo, and push. Git doesn't care which repo you originally got any given commit from. It "just works."