The OpeRATOR enzyme digests O-glycosylated proteins N-terminally to the serine (S) and threonine (T) glycosylation sites. OpeRATOR therefore facilitates mapping of O-glycosylation sites by LC-MS and as the digested products no longer bind to the GlycOCATCH resin, elution and sample preparation can be achieved in a single step. However, this requires the O-glycosylation sites to be spaced closely together for the OpeRATOR digestion to yield peptides of a size suitable for analysis by LC-MS/MS.