An oral ulcer is considered an ulcer that occurs on the mucous membrane of the oral cavity. Its treatment however poses a serious challenge as experts in this area of family health care are rare. In order to affirm this claim, questionnaire was designed and distributed to get up-to-date information concerning the ailment, particularly its treatment, causes, and solutions among others. An expert system seeks to mimic the functions and capabilities of a real life expert. This means that just as the expert is domain specific, the expert system as well is domain specific. It consists of a user interface, a knowledge-base, a fact-base, and an inference engine; all working together to mimic an expert. Bayesian inference was used in this work to factor in probability as it is well known that the fact that one is experiencing the symptoms of a disease does not absolutely mean that one has that disease. The domain of this expert system is oral ulcer and it focuses on four common oral ulcers which include: Cold sores, Gingivostomatitis, Herpangina as well as Neutropenia. The application which was designed with Object-Oriented Analysis and Design methodology was implemented with java programming language, provided on the Netbeans 8.0.2 IDE.