Mucin-type O-glycans decorate >80% of secretory and cell surface proteins and contribute to health and disease. However, dynamic alterations in the O-glycome are poorly understood because current O-glycomic methodologies are not sufficiently sensitive nor quantitative. Here we describe a novel isotope labeling approach termed Isotope-Cellular O-glycome Reporter Amplification (ICORA) to amplify and analyze the O-glycome from cells. In this approach, cells are incubated with Ac3GalNAc-Bn (Ac3GalNAc-[1H7]Bn) or a heavy labeled Ac3GalNAc-BnD7 (Ac3GalNAc-[2D7]Bn) O-glycan precursor (7 Da mass difference), which enters cells and upon de-esterification is modified by Golgi enzymes to generate Bn-O-glycans secreted into the culture media. After recovery, heavy and light Bn-O-glycans from two separate conditions are mixed, analyzed by MS, and statistically interrogated for changes in O-glycan abundance using a semi-automated approach. ICORA enables ~100–1000 fold enhanced sensitivity and increased throughput compared to traditional O-glycomics. We validated ICORA with model cell lines and used it to define alterations in the O-glycome in colorectal cancer. ICORA is a useful tool to explore the dynamic regulation of the O-glycome in health and disease.