The complexity of today's HPC systems increases as we move closer to the exascale goal, raising concerns about their sustainability. In an effort to improve their efficiency and effectiveness, more and more HPC installations are experimenting with fine-grained monitoring coupled with Operational Data Analytics (ODA) to drive resource management decisions. However, while monitoring is an established reality in HPC, no generic framework exists to enable holistic and online operational data analytics, leading to insular ad-hoc solutions each addressing only specific aspects of the problem. In this paper we propose Wintermute, a novel operational data analytics framework for HPC installations, built upon the holistic DCDB monitoring system. Wintermute is designed following a survey of common operational requirements, and as such offers a large variety of configuration options to accommodate these varying requirements. Moreover, Wintermute is based on a set of logical abstractions to ease the configuration of models at a large scale and maximize code re-use. We highlight Wintermute's flexibility through a series of case studies, each targeting a different aspect of the management of HPC systems, and demonstrate its small resource footprint.