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. Homohexamer. Belongs to the aspartate/ornithine carbamoyltransferase superfamily. OTCase family.