Writing pressure equilibrium constant expression for a reaction with a single solid product
The expression $\mathrm{K_p}$ is an expression of the equilibrium of partial pressures. So, it just includes the the partial pressure of the oxygen because that's the only gas.

$$\mathrm{K_P} = \frac{1}{\ce{P_{O2}}}$$

More information about the equilibrium constant $\mathrm{K_p}$ can be found here.