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 as a triacylglycerol synthase, making triacylglycerol (TG) from diolein and long-chain fatty acyl-CoA. Has no wax synthase activity to produce wax esters. 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 Optimum pH is 6.5. Glycerolipid metabolism; triacylglycerol biosynthesis. A possible member of the dormancy regulon. Induced in response to reduced oxygen tension (hypoxia), it is not induced by nitric oxide exposure. Belongs to the long-chain O-acyltransferase family. Lacks the conserved His residue in position 138 suggested to serve as a proton acceptor for this family, however this protein still has diacyglycerol O-acyltransferase activity in E.coli.