Catalyzes the terminal and only committed step in triacylglycerol synthesis by using diacylglycerol and fatty acyl CoA as substrates. Required for storage lipid synthesis. Upon expression in E.coli functions weakly as a triacylglycerol synthase, making triacylglycerol (TG) from diolein and long-chain fatty acyl-CoA. Has very weak wax synthase activity, incorporating palmityl alcohol into wax esters in the presence of palmitoyl-CoA. a 1,2-diacyl-sn-glycerol + an acyl-CoA = a triacyl-sn-glycerol + CoA (9Z)-octadecenoyl-CoA + di-(9Z)-octadecenoylglycerol = 1,2,3-tri-(9Z-octadecenoyl)-glycerol + CoA Glycerolipid metabolism; triacylglycerol biosynthesis. Induced in response to low levels of nitric oxide (NO). Belongs to the long-chain O-acyltransferase family.