Stearoyl-CoA desaturase that utilizes O(2) and electrons from reduced cytochrome b5 to introduce the first double bond into saturated fatty acyl-CoA substrates (PubMed:1978720, PubMed:7947684). Catalyzes the insertion of a cis double bond at the delta-9 position into fatty acyl-CoA substrates including palmitoyl-CoA and stearoyl-CoA (PubMed:1978720, PubMed:2674136). Required for the biosynthesis of membrane phospholipids, cholesterol esters and triglycerides (PubMed:1978720, PubMed:7947684, PubMed:16443825). Regulates fatty acid desaturation, that is, the ratio of unsaturated versus saturated fatty acyl chains, by competing with the acyltransferase STC1 for the common substrate C16:0-CoA. SCT1 sequesters C16:0-CoA into lipids, thereby shielding it from desaturation by OLE1 (PubMed:22323296). 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 + octadecanoyl-CoA = (9Z)-octadecenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O 2 Fe(II)-[cytochrome b5] + 2 H(+) + hexadecanoyl-CoA + O2 = (9Z)-hexadecenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O Expected to bind 2 Fe(2+) ions per subunit. The histidine box domains are involved in binding the catalytic metal ions. Belongs to the fatty acid desaturase type 1 family.