Epithelial ovarian cancer (EOC) is among the most prevalent and lethal gynecological malignancies, with a notably low five-year survival rate when diagnosed at advanced stages. High-grade serous ovarian carcinoma (HGSOC), the most common and aggressive EOC subtype, poses a particular challenge for early detection due to nonspecific or no visible symptoms in the early stages. Consequently, there is an urgent need for reliable biomarkers that can facilitate early and accurate diagnosis, especially for HGSOC. In our recent work, we identified a distinct glycosylation signature - or "glycocode" - on glycoproteins associated with extracellular vesicles (EVs) derived from HGSOC cells, underscoring their potential as disease-specific biomarkers. Building on this finding, we aimed to improve early detection, translational applicability, and precision diagnostics in HGSOC by characterizing the glycocode of proteins circulating in matched ascites and serum samples from a heterogeneous cohort of EOC patients. This cohort included benign controls (n=17), HGSOC (n=16), endometrioid carcinoma (n=11), clear cell carcinoma (n=7), and mucinous ovarian carcinoma (n=6). Using a glycoproteomics workflow combining TMT labeling, HILIC enrichment, and LC-MS/MS, we identified and quantified approximately 3,000 N-glycopeptides corresponding to 162 N-glycoproteins across all samples. The glycoproteome profiles of serum and ascites were largely comparable and dominated by sialylated complex-type glycans. Notably, HGSOC samples exhibited significantly higher levels of sialylated glycans in both serum and ascites compared to benign cases. Further validation using lectin assays with Sambucus nigra agglutinin (SNA), which selectively binds α2,6-linked sialic acids, confirmed elevated α2,6-sialylation in HGSOC, although this increase was not specific to this subtype. Site-specific glycoform analysis revealed HexNAc(4)Hex(5)NeuAc(2) as the predominant sialylated structure, significantly enriched in HGSOC and primarily carried by apolipoprotein B (APOB), histidine-rich glycoprotein (HRG), and α2-HS-glycoprotein (AHSG). Protein abundances remained unchanged, indicating that elevated sialylation reflects altered glycosylation rather than expression. Consistent site-specific changes in matched ascites further support the robustness of these markers. Together, these findings support the promise of glycosylation-based biomarkers, particularly α2,6-sialylation signatures, as minimally invasive diagnostic tools for the early detection of HGSOC.