Catalyzes the decarboxylation of oxaloacetate into pyruvate with high efficiency. Is also able to decarboxylate 3-methyloxaloacetate. Seems to play a role in maintaining cellular concentrations of bicarbonate and pyruvate. H(+) + oxaloacetate = CO2 + pyruvate Binds 1 Mg(2+) ion per subunit. Not inhibited by 3,3-difluoroxaloacetate. The turnover number for the oxaloacetate decarboxylation is equal to 7500 per second. There is nearly no change in activity from pH 5 to 10. Homotetramer; dimer of dimers. Belongs to the isocitrate lyase/PEP mutase superfamily. Oxaloacetate decarboxylase family.