In the various forms of interaction design, it is essential to analyze, understand, and predict human behavior. This is equally true with devices such as information systems that are meant to interact with people. The importance of these problems has inspired scientists to develop numerous approaches to investigate and explicate human actions. However, they have mainly been characterized by intuitive and folk psychological approaches to the human mentality in interaction. To improve the scientific foundations of design, we present here a psychology-based approach to collecting user knowledge, as well as a related design practice. The former can be called user psychology and the latter the action-oriented design. User psychology is an approach that applies psychological knowledge and methods to analyzing and solving interaction design problems. It works to develop explanatory design practices so that it is possible to say on which psychological grounds one design alternative is better than another or why a solution is ineffective. One step toward improving explanatory design practices is to develop effective design ontologies to manage the design processes. Here, we discuss the nature of user psychological knowledge and analyze the process of developing respective ontological solutions.