Catalyzes the decarboxylation of orotidine 5'-monophosphate (OMP) to uridine 5'-monophosphate (UMP). H(+) + orotidine 5'-phosphate = CO2 + UMP Pyrimidine metabolism; UMP biosynthesis via de novo pathway; UMP from orotate: step 2/2. Homodimer. Belongs to the OMP decarboxylase family. Type 1 subfamily.