First, setup Octopress following its instructions. Then, add an org-mode export project to your .emacs:
(setq org-publish-project-alist '(("blog" . (:base-directory "~/blog/source/org_posts/" :base-extension "org" :publishing-directory "~/blog/source/_posts/" :sub-superscript "" :recursive t :publishing-function org-publish-org-to-html :headline-levels 4 :html-extension "markdown" :body-only t))))
Now modify the Octopress Rakefile: Generate a new post like normal
$ rake new_post["Title"]
Edit the org_posts/DATE-title.org file to write your post and hit C-c C-e F to export it.