Annex I of the International Convention for the Prevention of Pollution from Ships (MARPOL) regulates the prevention of oil pollution from vessels and constitutes a mandatory requirement for the operations of oil tankers such as the MT. Rosa XVIII (3,694 Gross Tonnage, PT. Mammiri Line). However, the implementation of these regulations often encounters various technical and operational obstacles in the field. This study aims to identify and analyze the factors causing compliance issues regarding MARPOL Annex I on board the MT. Rosa XVIII, utilizing a cause-and-effect analysis—specifically the "Fishbone Diagram" based on the 6M framework (Man, Machine, Method, Material, Measurement, and Milieu). A descriptive-qualitative research method was employed, involving observations, crew interviews, and a review of vessel specifications and operational records. The analysis reveals that the primary obstacles stem from limited crew competence (Man), maintenance issues with oil separation equipment (Machine, such as the Oily Water Separator), inadequacies in standard operating procedures (Method), errors in filling out the Oil Record Book (Material), inaccuracies in waste level measuring instruments (Measurement), and environmental and operational weather conditions (Milieu). This study offers strategic recommendations to enhance the effectiveness of safety management and marine pollution prevention within the shipping company.