When someone is lactose intolerant, it means its intestine cannot absorb lactose, which will stay in the lumen and be consumed by intestinal bacteria and produced gases, that's why intolerant lactose people have flatulence and abdominal pain while eating food with lactose in it. Let's go back to the question, before being consumed, lactose in the lumen intestine will attract water from the enterocytes to its side by oncotic pressure.
Do not confound oncotic pressure (which concerns macromolecules like sugar or proteins) and osmotic pressure (which concern micromolecules like small ions (sodium potassium and chloride...)).
