A lubricating oil for refrigerating machines which comprises an oxygenic compound as a base oil and which, when incorporated into a chlorine-free C1 refrigerant in an amount in the range of 3 to 50 wt.% based on the sum of these, is compatible therewith in any of the concentrations at a temperature of -10°C or lower; and a hydraulic composition for refrigerating machines which comprises (A) a chlorine-free C1 refrigerant and (B) the lubricating oil. In the composition, the oil has excellent compatibility with the chlorine-free C1 solvent, especially difluoromethane. The composition is excellent in antiwearing properties, lubricating properties, stability, etc.