Catalyzes the terminal and only committed step in triacylglycerol synthesis by using diacylglycerol and fatty acyl CoA as substrates (PubMed:18704537). Highly expressed in epithelial cells of the small intestine and its activity is essential for the absorption of dietary fats. In liver, plays a role in esterifying exogenous fatty acids to glycerol, and is required to synthesize fat for storage (By similarity). Also present in female mammary glands, where it produces fat in the milk (PubMed:18704537, PubMed:15342525). May be involved in VLDL (very low density lipoprotein) assembly (By similarity). In contrast to DGAT2 it is not essential for survival (By similarity). Functions as the major acyl-CoA retinol acyltransferase (ARAT) in the skin, where it acts to maintain retinoid homeostasis and prevent retinoid toxicity leading to skin and hair disorders (By similarity). Exhibits additional acyltransferase activities, includin acyl CoA:monoacylglycerol acyltransferase (MGAT), wax monoester and wax diester synthases (By similarity). Also able to use 1-monoalkylglycerol (1-MAkG) as an acyl acceptor for the synthesis of monoalkyl-monoacylglycerol (MAMAG) (By similarity). a 1,2-diacyl-sn-glycerol + an acyl-CoA = a triacyl-sn-glycerol + CoA all-trans-retinol + an acyl-CoA = an all-trans-retinyl ester + CoA (9Z)-octadecenoyl-CoA + 2-(9Z-octadecenoyl)-glycerol = 1,2-di-(9Z-octadecenoyl)-sn-glycerol + CoA (9Z)-octadecenoyl-CoA + 1,2-di-(9Z-octadecenoyl)-sn-glycerol = 1,2,3-tri-(9Z-octadecenoyl)-glycerol + CoA all-trans-retinol + hexadecanoyl-CoA = all-trans-retinyl hexadecanoate + CoA (9Z)-octadecenoyl-CoA + 1-O-(9Z-octadecenyl)-glycerol = 1-O-(9Z-octadecenyl)-mono-(9Z-octadecenoyl)-glycerol + CoA (9Z)-octadecenoyl-CoA + 1-O-(9Z-octadecenyl)-mono-(9Z-octadecenoyl)-glycerol = 1-O-(9Z-octadecenyl)-2,3-di-(9Z-octadecenoyl)glycerol + CoA (9Z)-octadecenoyl-CoA + 1-(9Z-octadecenoyl)-glycerol = 1,2-di-(9Z-octadecenoyl)-glycerol + CoA (9Z)-octadecenoate + 1,2-di-(9Z-octadecenoyl)-glycerol + H(+) = 1,2,3-tri-(9Z-octadecenoyl)-glycerol + H2O (9Z)-octadecenoyl-CoA + 1-octadecanoyl-2-(5Z,8Z,11Z,14Z-eicosatetraenoyl)-sn-glycerol = 1-octadecanoyl-2-(5Z,8Z,11Z,14Z)-eicosatetraenoyl-3-(9Z)-octadecenoyl-sn-glycerol + CoA hexadecane-1,2-diol + 2 hexadecanoyl-CoA = 1,2-O,O-dihexadecanoyl-1,2-hexadecanediol + 2 CoA hexadecane-1,2-diol + hexadecanoyl-CoA = 2-hydroxyhexadecyl hexadecanoate + CoA 2-(9Z-octadecenoyl)-glycerol + hexadecanoyl-CoA = 1-hexadecanoyl-2-(9Z-octadecenoyl)-sn-glycerol + CoA 1,2-di-(9Z-octadecenoyl)-sn-glycerol + hexadecanoyl-CoA = 1,2-di-(9Z)-octadecenoyl-3-hexadecanoyl-sn-glycerol + CoA hexadecan-1-ol + hexadecanoyl-CoA = CoA + hexadecanyl hexadecanoate 13-cis-retinol + hexadecanoyl-CoA = 13-cis-retinyl hexadecanoate + CoA (9Z)-octadecenoyl-CoA + 1,3-di-(9Z-octadecenoyl)-glycerol = 1,2,3-tri-(9Z-octadecenoyl)-glycerol + CoA (9Z)-octadecenoyl-CoA + 2,3-di-(9Z)-octadecenoyl-sn-glycerol = 1,2,3-tri-(9Z-octadecenoyl)-glycerol + CoA Lipid metabolism; glycerolipid metabolism. Homodimer or homotetramer; both forms have similar enzymatic activities. The disordered N-terminal region is required for the diacylglycerol O-acyltransferase activity and may regulate enzymatic function via its interaction with the MBOAT fold. The MBOAT fold forms a reaction chamber in the endoplasmic reticulum membrane that encloses the active sites. The reaction chamber has a tunnel to the cytosolic side and its entrance recognizes the hydrophilic CoA motif of an acyl-CoA molecule. The chamber has separate entrances for each of the two substrates, acyl-CoA and 1,2-diacyl-sn-glycerol. Lys-232 is associated with higher milk fat content. Belongs to the membrane-bound acyltransferase family. Sterol o-acyltransferase subfamily.