The concept of "evidence-based policy making" (EBPM) has been gaining currency over the past decade and is being promoted by governments around the world as a mechanism for developing more effective and efficient policies and programs based on scientific evidence. Moreover, the advent of open government data, together with the increasing adoption of indicators and networked, mobile sensors that enable systematic monitoring, is making evidence-based policy-making increasingly viable - particularly within the context of "Smart Cities". However, one of the key challenges associated with EBPM is the integration and analysis of a wide range of data and knowledge (including policies, programs, targets, indicators) that are heterogeneous, unstructured and dynamic. This paper proposes a high level ontology that provides a consistent, systematic approach to evidence-based policy making within Smart Cities and helps bridge the gap between the monitoring agencies who generate the indicators and the policy-makers who consume it. The proposed EBPM ontology is evaluated in the context of an environmental case study. Challenges associated with the proposed ontological framework are identified and potential future research directions highlighted.