Illustrated Glossary of Organic Chemistry

Oligosaccharide: A molecule containing between two and ten monosaccharides. When the oligosaccharide contains two monosaccharides it is a disaccharide. When the oligosaccharide contains more than ten monosaccharides it is a polysaccharide.

Lactose contains two monosaccharides (glucose and galactose), so it is both a disaccharide and an oligosaccharide.

Cellulose is a polysaccharide because it contains thousands of glucose subunits in each cellulose molecule.