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. Homotrimer. Belongs to the aspartate/ornithine carbamoyltransferase superfamily. OTCase family. Extended N-terminus.