By default, Octopress deploys your posts, categories and archives to a /blog/ subdirectory. Here is how to remove that :

• replace all /blog occurences by / in _config.yml
• move source/blog/archives to source/ (you can delete blog/)
• edit source/_includes/custom/navigation.html. Replace /blog/archives by /archives
• edit source/index.html. Replace /blog/archives by /archives

That’s it.