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 biosynthesis; L-arginine biosynthesis; L-arginine from L-ornithine and carbamoyl phosphate: step 1/3. Belongs to the aspartate/ornithine carbamoyltransferase superfamily. OTCase family. Lacks the conserved threonine and leucine residues in positions 66 and 276, respectively, which are part of the carbamoylphosphate and ornithine binding sites; they are replaced by a leucine and a methionine residue, respectively.