Hello,
well V(OL) is equal Vm, that region where both Pmos and Nmos are in Saturation..
if you want to find Vm simple draw the schematic of an Inverter with ratio PUN/PDN= 2:1
PLot do the DC analysis(sweep the input voltages)... the point where both input and output intersects is the VM its is usually taken as VDD/2. i.e if VDD = 5v than Vm will be equal to 2.5 and so is the VOL.. good luck...