A Mars sample return mission has been proposed where oxygen for the return propellant would be produced using the Martian atmosphere. A mass analysis of an in-situ resource utilization (ISRU) oxygen plant shows that significant reductions in landed mass and mission costs can be attained by using methane carried from Earth and oxygen produced on Mars as propellents for the return trip. The metrics for the mass analysis are based on laboratory data collected from proof-of-concept, scale-up, and long-duration experiments. Using existing technology and current performance levels, it is shown that an ISRU plant capable of producing 7 kg/ day of oxygen requires an estimated mass of 108 kg and consumes 2.9 kW of electrical power. A 2.5 kg/day oxygen production facility has a mass of 64 kg and requires 1.1 kW of electrical power.