Abstract: The refrigerating machine oil of the invention includes an ester of a polyhydric alcohol and a fatty acid, wherein the molar ratio of C4-C6 fatty acid and C7-C9 branched fatty acid in the fatty acid is between 15:85 and 90:10, the C4-C6 fatty acid includes 2-methylpropanoic acid, and the ratio of the total C4-C6 fatty acid and C7-C9 branched fatty acid in the total fatty acids composing the ester is at least 20 mol %. The working fluid composition for a refrigerating machine according to the invention comprises the refrigerating machine oil, a difluoromethane refrigerant and/or an unsaturated fluorinated hydrocarbon refrigerant.