OTP is an acronym that stands for One True Pairing.
The phrase is used to describe a relationship between two characters that a fan believes is the only "real" relationship allowed or possible. Adherence to the OTP may range from the rabid (where hate will be directed toward all other 'ships), to a general statement of the fan's preferred reading or writing preferences.
In Supernatural, there is a noted absence of shipping wars, due to the presence of only two main characters - Sam/Dean - if a fan is comfortable with Wincest. From Season Four onwards there have been some conflicts between fans of Sam/Dean and those of the new pairing of Dean/Castiel.