1. Switch themes
2. Keep the theme and use a workaround.
Not all blog themes are designed to display full posts on the front page of the blog. Please read the theme description here > http://theme.wordpress.com/themes/origin/
The Origin theme is behaving exactly the way it is designed to behave, which is excerpts only on the front page of the blog but Origin does have an active excerpt box below the post editor, so you can control the length of the excerpt displayed. This is the workaround you can use to override excerpt display on the front page.
You compose your whole post first and before you publish it you switch to the HTML (Text) editor, copy the code, paste it into the Excerpt box, and publish the post. http://en.support.wordpress.com/splitting-content/excerpts/#creating-excerpts
This will have to be done on every post.