Atomic oxygen (AO) fluence impinging on a spacecraft in low earth orbit is predicted based on a designed scenario for each mission. The results are then analyzed to determine its effect on the performance of the spacecraft. This task involves a large data base of spacecraft surface material properties and space environment, repeated application of AO fluence computation models, and judicious interpretation of the results, and appears to be an ideal application for an expert system. An atomic oxygen expert system is developed to integrate the various steps of the task into a user-friendly software package that asks a few questions at the beginning and gives a report at the end. The knowledge base is the brain of the system. The current knowledge base consists of files of material reaction efficiencies from literature and experiments, and the solar activity forecast from NASA reports. The solar activity data cover the period from 1980 to 2008. The knowledge base needs to be expanded and updated periodically. The forward-chaining inference engine of the expert system requires just a few rules, governing the time and the orbital position in the AO fluence computation. The analytical power is provided by the existing neutral atmospheric model and an orbital mechanics computer code. The output includes a report of the total AO fluence and the predicted mass loss. If a threshold value is provided, a pass or fail flag is also shown. The effects, such as Δα and Δε changes, caused by the predicted fluence are reported according to the specific material. Its format varies to suit the application. Examples of application from Space Station Freedom and Long Duration Exposure Facility are included.