Answer
$653\;\rm J/kg\cdot K$
Work Step by Step
The oxygen’s specific heat at constant volume is $c_{\rm V}$ which is the amount of energy required to raise the temperature of a unit mass of a substance by one degree Celsius while keeping the volume constant.
We know that oxygen’s specific heat at constant volume, from Table 17.4, is given by
$$c_{\rm V}=20.9\;\rm J/mol \cdot K$$
and we need to find it in $\rm J/kg \cdot K$ where we know that the atomic mass of the oxygen atom is 16 g/mol, so the oxygen gas of two oxygen atoms is 32 g/mol which is 0.032 kg/mol.
Multiplying the equation above by the reciprocal of 0.032 kg/mol.
$$c_{\rm V}=\rm\dfrac{20.9\;\rm J}{1\;mol \cdot K}\left[\dfrac{1\;mol}{0.032\;kg}\right]$$
$$c_{\rm V}=\color{red}{\bf 653}\;\rm J/kg\cdot K$$
