The ability to detect and monitor oil spills at sea is becoming increasingly important due to the high demand of oil-based products. Remote sensing frameworks have been proven to give accurate results in case of major events; nonetheless, also medium and micro oil spills have their own importance, especially in protected areas that deserve special attention. In this paper, we propose a monitoring framework based on the collection of in situ observations and on their integration with remote sensing in order to fill out existing observational gaps. In particular, besides the data collected by special monitoring devices, in situ observations include volunteered geographical information as an additional source of valuable data. Oil spill sights, notified by volunteers through a specially-designed app, are integrated in the monitoring system and therein processed together with remote sensing data in order to proactively detect anomalous events and produce alerts. Field operational tests in two areas demonstrate the technical validity of the approach, while users' reception testifies to its potential in raising people's awareness on marine pollution.