A methodology to conduct the Response Gap Analysis (RGA) for offshore oil spill emergency activities was developed, able to identify, for a given area and a given time period, if the response, which depends on the combination of the environmental factors affecting the performance of the contingency actions, is favourable, impaired, or ineffective. The necessity for offshore oil and gas operators to perform a RGA derives from the provisions of the Directive 2013/30/EU on safety of offshore oil and gas operations. In this Directive, “oil spill response effectiveness” means the effectiveness of spill response systems in combating an oil spill, on the basis of the “analysis of the frequency, duration, and timing of environmental conditions that would preclude a response. Furthermore, the assessment of oil spill response effectiveness is to be expressed as a percentage of time that such conditions are not present and is to include a description of the operating limitations placed on the installations concerned as a result of that assessment”. Firstly, a gap analysis was conducted, aimed at highlighting the state of the art of RGA. The various literature reports on RGA are based on different environmental factors, as well as on different response strategies. A detailed analysis of the available information allowed identifying 9 environmental factors to be included in RGA: the wind speed, the wave height, the air temperature, the wind chill, the superstructure icing, the daylight / darkness conditions, the horizontal visibility, the cloud ceiling, and the sea ice coverage. In addition, also the shoreline distance and the bathymetry, which are location specific factors, could impair the response. Thus, a total number of 11 “environmental factors” is taken into account in the new RGA methodology. The definition of threshold values to be applied to each environmental factor was also addressed. In literature, usually two limit values are adopted and three zones are identified: a green zone, where the factor is favourable to the response; a yellow zone, where it is marginal, and a red zone, where it is unfavourable. Technical reports were analysed in order to retrieve the limit values of the environmental factors mentioned. Thus, a set of threshold values was proposed for mechanical recovery, application of dispersants by vessel and by aircraft, and in-situ burning. As a further step, the criteria adopted to combine the environmental factors in order to evaluate the effectiveness of the response were investigated. All the most recent studies identify for the response a green zone for the favourable response, a yellow zone corresponding to the impaired response, and a red one, where the response is ineffective. This criterion is adopted in the RGA method developed in the present study. The RGA methodology was implemented in a software code, which was applied to a case study area, allowing the validation of the RGA procedure.