SUMMARY: Oligator is software designed to assist scientists in their exploration of MS/MS experiments, especially for oligosaccharides bearing unreferenced chemical substitutions. Through a graphical interface, users have the total flexibility to build a candidate glycan structure and produce the corresponding theoretical MS/MS spectrum in accordance with the usual ion nomenclature. The structural information is saved using standard notations, in text format, which facilitates the capitalization and exchange of data as well as any other processing of the information. AVAILABILITY AND IMPLEMENTATION: Source code and user manual are freely available at https://github.com/vlollier/oligator. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.