The objective of this paper is to present a conceptual architecture of a Building Energy Management Open Source Software (BEMOSS) platform. BEMOSS is an open source operating system that is expected to improve sensing and control of equipment in small- and medium-sized commercial buildings, reduce energy consumption and help implement demand response (DR). It aims to offer: scalability, robustness, plug and play, open protocol, interoperability, cost-effectiveness, as well as local and remote monitoring. In this paper, four essential layers of BEMOSS software architecture — namely User Interface, Application and Data Management, Operating System and Framework, and Connectivity layers — are presented. A laboratory test bed to demonstrate the functionality of BEMOSS located at the Advanced Research Institute of Virginia Tech is also briefly described.