The present invention provides synthetic Moraxella catarrhalis lipooligosaccharide (LOS)-based oligosaccharides and conjugates containing various M. catarrhalis serotype-specific oligosaccharide antigens or various core M. catarrhalis oligosaccharide structures or motifs corresponding to one or more of the three major serotypes and/or members within a given serotype. The oligosaccharides may be synthesized by a chemical assembly methodology relying on a limited number of monosaccharide and disaccharide building blocks. The invention further provides M. catarrhalis LOS-based immunogenic and immunoprotective compositions and antibodies derived therefrom for diagnosing, treating, and preventing infections caused by M. catarrhalis.