Reversibly catalyzes the transfer of the carbamoyl group from carbamoyl phosphate (CP) to the N(epsilon) atom of ornithine (ORN) to produce L-citrulline, which is a substrate for argininosuccinate synthetase, the enzyme involved in the final step in arginine biosynthesis. carbamoyl phosphate + L-ornithine = H(+) + L-citrulline + phosphate Inhibited by the bisubstrate delta-N-phosphonoacetyl-L-ornithine (PALO). Optimum pH is 6.5 (at 55 degrees Celsius). Extreme thermal stability. It maintains about 50% of its activity after heat treatment for 60 min at 100 degrees Celsius. Amino-acid biosynthesis; L-arginine biosynthesis; L-arginine from L-ornithine and carbamoyl phosphate: step 1/3. Homododecamer (tetramer of trimers). The monomer folds into two domains of similar size. The N-terminal domain is the carbamoylphosphate-binding domain; ornithine binds to the C-terminal domain. It interacts physically with carbamoyl-phosphate synthetase (CKase), forming a channeling cluster for carbamoyl phosphate. This prevents the thermodenaturation of carbamoyl phosphate, an extremely thermolabile and potentially toxic metabolic intermediate (PubMed:9501170). Belongs to the aspartate/ornithine carbamoyltransferase superfamily. OTCase family.