Catalysis of the reaction: diolein + mono-olein = triolein + glycerol. Mono-olein, also known as mono-oleoylglycerol, is the monoglyceride formed from oleic acid, 9-octodecenoic acid; diolein is also known as dioleoylglycerol, and triolein as trioleoylglycerol and olein.