According to the Charles' law of gases, the volume occupied by a gas and the temperature are directly proportional to each other, provided the pressure remains constant.
Thus, at constant pressure conditions,
V `alpha` T
where, V and T represent the volume and temperature of gases. In other words,
`V/T = constant`
or, `V_1/T_1 = V_2/T_2`
where the notations 1 and 2 represent older conditions (volume and temperature) and new conditions, respectively.
Here, V1 = 50.0 liters, T1 = 20.0 degrees Celsius and V2 = 5.0 liters
Thus, T2 = (V2/V1) T1 = (5/50) x 20 = 2.0 degrees Celsius.
Thus, at constant pressure, the temperature of the oxygen will have to be decreased to 2 degrees Celsius (from an initial temperature of 20 degrees Celsius) to be able to compress it from 50 liters to 5 liters.
Hope this helps.