Reversibly catalyzes the transfer of the carbamoyl group from carbamoyl phosphate (CP) to the N(epsilon) atom of ornithine (ORN) to produce L-citrulline. carbamoyl phosphate + L-ornithine = H(+) + L-citrulline + phosphate Amino-acid degradation; L-arginine degradation via ADI pathway; carbamoyl phosphate from L-arginine: step 2/2. Belongs to the aspartate/ornithine carbamoyltransferase superfamily. OTCase family.