Microbial contamination can occur during long-term storage of domestic heating oil (DHO), thus biofouling the fuel and equipment. In this study, microorganisms in domestic heating oil storage tanks were identified, and the factors that promote growth in DHO storage tanks were examined. Notably, obligate aerobic but not strict anaerobic microbes were detected, suggesting the existence of an oxic environment. The measured and calculated diffusion rates of oxygen in the oil support the finding that oxygen limitation in a storage tank is unlikely. Fatty acid methyl ester (FAME) fostered microbial growth in DHO tanks, and fewer microorganisms were required to initiate growth in FAME-blended fuels. The results are discussed in the context of minimizing microbial contamination during long-term storage of FAME blends in domestic heating oil applications.