Methods for the preparation of oligosaccharide products from polysaccharide starting materials are disclosed. The methods include: hydrolyzing a glucosamine-containing polysaccharide starting material, such as heparin or heparosan, under conditions sufficient to form an oligosaccharide intermediate (e.g., a GlcN-IdoA disaccharide intermediate or a GlcA-GlcN disaccharide intermediate), and converting the oligosaccharide intermediate to the oligosaccharide product. Conversion of the oligosaccharide intermediates to the oligosaccharide products may include one or more esterification, acylation, epimerization, protection, and deprotection steps. Preparation of higher-order oligomers is described, as well as methods for selective oligosaccharide sulfation.