This paper describes a novel tool for eliciting user requirements early in the design process of mobile applications. The “Mobile Oracle”, as we have called it, is intended to help developers and designers obtain a better understanding of what the user wants at different points in space and time. It is an extension of a lo-fi version of the well-established Wizard of Oz technique, but it adds an “on demand” component to force users to explicitly request the information they need. The technique has been tested in an investigation involving 15 users (sighted, visually impaired, and elderly). Our preliminary results show it to generate valuable information concerning the ways people ask about directions and distances, as well as the services they would like to have in future mobile applications. Categories and Subject Descriptors D.2.1 [Requirements/Specifications] General Terms Design, Human Factors Keywords Design, on demand, wizard of oz, user requirements