Nitric oxide (NO) is a free radical which is produced from a wide variety of cells and tissues in the human body. NO is involved in the regulation of many physiological processes, such as vascular relaxation, neurotransmission, immune regulation, and cell death. NO is generated by nitric oxide synthase (NOS), which has three identified isoforms: neuronal type NOS (nNOS), endothelial type NOS (eNOS), and inducible type NOS (iNOS). Different isoforms are expressed depending on the organs, tissues, and cells, and investigation of the types and functions of enzymes expressed in various tissues is underway. The oral cavity is a space in which marked changes have been detected in NO levels, and each tissue is constantly influenced by NO. NO is a component of saliva and is produced by oral bacteria in the oral cavity and released by NOS expressed in oral mucosa. NOS isoforms expressed under normal conditions differ among the oral organs. In addition, the overexpression of NOS was involved in carcinogenesis and tumor growth progression. This review summarized the expression of NOS and functions of NO in oral cavity organs, and their roles in diseases and the influences of treatments.