LeMill is an open source OER repository where the emphasis has been placed on designing a service to meet the actual needs of teachers preparing for classes. The development of LeMill has utilized open, collaborative, and iterative design methods and many features have been refined or redesigned during the process. Emphasis on design work has helped LeMill avoid and fix problems that generally pester OER repositories because of their origins as learning object repositories. The authors recognize that LeMill, as an open source project, has had the rare benefit of a long, structured dissemination phase incorporating actual teacher training. Even when developers and designers try to keep teachers in mind, actual behavioral patterns and needs appear only after the service has been in use. Therefore systems should initially be flexible enough to allow changes resulting from new findings.