Satire v. Fox News

Today in class we saw a short documentary about how fox news tried to essentially censor a satirical book for being critical of Bill O'Reilly. While fox was clearly in the wrong for trying to censor satire, what is the line between libel and satire. Legally the difference is saying something untrue and for the purpose of hurting someone is libel, but occasionally, you can create something intended as satire and end up hurting somebody. If a satirical article pops up on the internet and people don't realize it is satirical, does the creator have an obligation to tell the world it is satire?