Most cancers of the mouth or oral cavity occur in the floor of the mouth and tongue. However, they may also occur in the cheek lining, gums, lips, and upper or lower jaw. There is also an area right behind the mouth known as the oropharynx. Oropharyngeal cancer refers to any type of cancer that occurs behind the tongue, tonsils, and throat tissue. This makes up about one-third of all oral cancer cases.