consider the following reversable reaction C (s) + O2 (g) arrow thing, forgot what its called CO2 (g). what is the equilibrium constant expression for the given system?

Equilibrium constant expression equals (pressure of co2) / (pressure of O2). you don't include c in the expression because solids are never included and if pressure is not given and the volume is 1L use the number of moles given rather than the pressure.