Original content (OC) is any media that hasn't been posted to an imageboard before. Usually this means content that was made by the uploader himself. Rarely is the uploader also someone very popular, in which case the original content would be seen on the content owner's popular website.
When the term original content is used in reference to pornography or camwhores, it usually means that the media is fresh or new, and hasn't been posted or uploaded multiple times to every site out there. If the source of the media isn't the camwhore itself, it comes first hand from the person who took the pictures or captured the video.
Despite the "no original content on 4chan" meme, /b/ is surprisingly able to create OC with relative ease. Some of the OC becomes small scale in-jokes exclusive to /b/; some traveling to the furthest reaches of the Internet. Unfortunately, /b/ is often left in the wilderness, while less offensive websites take the glory, such as LOLCats and ORLY Owl spin-offs.
Quite a bit of original content comes from the use of Rule #34. Occasionally, artist-residents of /b/ have made request threads and populated them with the artistic demands of Anonymous. One such artist was Kristal, who regularly had mini-contests on /b/, with the 10th post in the thread being the winner of having their request drawn by Kristal, and the 10th post after that drawing, and so on. Kristal's work used to be located at http://kristal.smonson.com/gallery/v/4chan/.