The ratio of the mass of copper to the mass of oxygen in the compound can be determined by dividing the mass of copper by the mass of oxygen:

$$Ratio = \frac {Mass \ of \ Copper} {Mass \ of \ Oxygen } =\frac {64 \ g}{16 \ g} =4$$

Therefore, the ratio of the mass of copper to the mass of oxygen in the compound is 4 : 1.