This paper describes the conceptualization, design and development of an instrument to ascertain the impacts of the adoption and integration of open educational practices in teaching and learning. We have called it the Open Educational Practices Impact Evaluation Index (OEP-IE Index). When administered at regular intervals over a period of time, this instrument helps to capture behavioral shifts in the perceptions, perspectives and practices of adopters of open educational practices in any educational setting. The development of this instrument is a product of the work that is being undertaken as part of the ROER4D project at the Open University of Sri Lanka to evaluate the impacts of the adoption and integration of Open Educational Resources (OER) and Open Educational Practices (OEP) by teachers in Sri Lankan schools. The development of this instrument is currently in the pilot stages.