Human fibrinogen is a 340 kDa, soluble plasma glycoprotein composed of paired sets of three subunits (α, β, γ). The protein plays a crucial role in protecting the vascular network against the loss of blood after tissue injury. The beta and gamma subunits each contain one N-glycosylation site, each of which is occupied by a biantennary N-glycan. So far O-linked oligosaccharides have rarely been described. Here, we make use of tryptic- and proteinase K-generated fibrinogen glycopeptides for the detailed analysis of the protein's O-glycosylation by combining information obtained from both one- and two-dimensional nanoLC-ESI-ion trap (IT)-MS approaches. Glycopeptides were analyzed by ion trap-MS/MS which displayed fragmentations of glycosidic linkages and some peptide backbone cleavages. MS3 spectra of the generated O-glycopeptides showed cleavages of the peptide backbone and provided essential information on the peptide sequence. The previously reported N-glycan attachment sites of human fibrinogen could be confirmed. Moreover, we describe seven novel O-glycosylation regions in human fibrinogen, all occupied by a monosialylated T-antigen. Our findings may help to improve the general understanding of human fibrinogen in the blood clotting process.