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
If you do not see an excerpt box then go to screen options on the top right hand corner of the post editor screen and include:
Post – Likes and Shares, Format*, Categories, Tags, Excerpt, Send Trackbacks, Discussion, Comments**, Slug, Author, Revisions, Writing Helper
This will have to be done on every post.