Catalyzes the transfer of a ribosyl phosphate group from 5-phosphoribose 1-diphosphate to orotate, leading to the formation of orotidine monophosphate (OMP). Catalyzes the decarboxylation of orotidine monophosphate (OMP) to uridine monophosphate (UMP). H(+) + orotidine 5'-phosphate = CO2 + UMP diphosphate + orotidine 5'-phosphate = 5-phospho-alpha-D-ribose 1-diphosphate + orotate Pyrimidine metabolism; UMP biosynthesis via de novo pathway; UMP from orotate: step 1/2. Pyrimidine metabolism; UMP biosynthesis via de novo pathway; UMP from orotate: step 2/2. In the N-terminal section; belongs to the OMP decarboxylase family. Type 2 subfamily. In the C-terminal section; belongs to the purine/pyrimidine phosphoribosyltransferase family.