Condensed highlights from a decade of Marine Mammal Observer (MMO) and Passive Acoustic Monitoring (PAM) projects carried out from stationary and on-tow Oil & Gas (O&G) exploration jack-up drilling-rigs and production platforms is presented. The majority of work was undertaken as routine monitoring and mitigation permit requirements, but a partial aim was to investigate the hypothesis that, because of a potential rig-as-a-reef effect, echolocation activity of harbour porpoises (Phocoena phocoena) is likely to be higher at offshore installations compared with the open sea. Porpoise activity was monitored visually and acoustically using MMOs, real-time PAM, and a brief, controlled experiment using T-PODs – autonomous, static, echolocation-click detectors. Throughout the last decade, porpoises were detected visually, acoustically, and consistently at varying levels, at all offshore locations undergoing routine operational activities. The controlled T-POD experiment gave reasonable evidence to support the hypothesis that porpoises may target offshore installations compared to the open sea, but further long-term, replicated and controlled visual and acoustic experiments using recently optimised mooring techniques are required to ‘put the theory to bed’. In conclusion, this paper puts forward the notion that decommissioning of O&G installations in the North Sea may remove valuable porpoise foraging habitat and thus may have implications for the long-term survival of this listed and threatened species.