Catalyzes the formation of fatty acid-cholesterol esters, which are less soluble in membranes than cholesterol (PubMed:16154994, PubMed:16647063, PubMed:9020103, PubMed:32433614, PubMed:32433613, PubMed:32944968). Plays a role in lipoprotein assembly and dietary cholesterol absorption (PubMed:16154994, PubMed:9020103). Utilizes oleoyl-CoA ((9Z)-octadecenoyl-CoA) preferentially as susbstrate: shows a higher activity towards an acyl-CoA substrate with a double bond at the delta-9 position (9Z) than towards saturated acyl-CoA or an unsaturated acyl-CoA with a double bond at the delta-7 (7Z) or delta-11 (11Z) positions (PubMed:11294643, PubMed:32433614). a long-chain fatty acyl-CoA + a sterol = a sterol ester + CoA an acyl-CoA + cholesterol = a cholesterol ester + CoA (9Z)-octadecenoyl-CoA + cholesterol = cholesteryl (9Z-octadecenoate) + CoA cholesterol + hexadecanoyl-CoA = cholesteryl hexadecanoate + CoA cholesterol + octadecanoyl-CoA = cholesteryl octadecanoate + CoA (9Z,12Z)-octadecadienoyl-CoA + cholesterol = cholesteryl (9Z,12Z)-octadecadienoate + CoA (5Z,8Z,11Z,14Z)-eicosatetraenoyl-CoA + cholesterol = cholesteryl (5Z,8Z,11Z,14Z)-eicosatetraenoate + CoA (9Z)-hexadecenoyl-CoA + cholesterol = cholesteryl (9Z)-hexadecenoate + CoA (11Z)-octadecenoyl-CoA + cholesterol = cholesteryl (11Z)-octadecenoate + CoA (7Z)-octadecenoyl-CoA + cholesterol = cholesteryl (7Z)-octadecenoate + CoA Cholesterol O-acyltransferase activity is inhibited by nevanimibe. Homotetramer; composed of two homodimers (PubMed:32433614, PubMed:32433613). Interacts with UBIAD1 (PubMed:23169578). Highly activated by the presence of cholesterol. Each protomer consists of 9 transmembrane segments, which enclose a cytosolic tunnel and a transmembrane tunnel that converge at the predicted catalytic site: acyl-CoA enters the active site through the cytosolic tunnel, whereas cholesterol enters from the side through the transmembrane tunnel. Belongs to the membrane-bound acyltransferase family. Sterol o-acyltransferase subfamily.