Mannosyltransferase that transfers an alpha-D-mannosyl residue from GDP-mannose into lipid-linked oligosaccharide, forming an alpha-(1->2)-D-mannosyl-D-mannose linkage. Required for the attachment of the third mannose residue of O-linked saccharides. Protein modification; protein glycosylation. Present with 13000 molecules/cell in log phase SD medium. Belongs to the glycosyltransferase 15 family.