RuBisCo has a much greater affinity for CO2 than for O2. It is the relative concentration of O2 and CO2 that determines which of the two will bind to the enzyme. In C3 plants some O2 does bind to RuBisCo and hence CO2 fixation is decreased.
In C4 plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO2 at the enzyme site. This takes place when the C4 acid from the mesophyll is broken down in the bundle cells to release CO2 . This results in increasing the intracellular concentration of CO2. In turn, this ensures that the RuBisCo functions as a carboxylase minimising the oxygenase activity.