alpha-Synuclein (alpha-Syn) is an intrinsically disordered protein which self-assembles into highly organized beta-sheet structures that accumulate in plaques in brains of Parkinson's disease patients. Oxidative stress influences alpha-Syn structure and selfassembly; however, the basis for this remains unclear. Here we characterize the chemical and physical effects of mild oxidation on monomeric alpha-Syn and its aggregation. Using a combination of biophysical methods, small-angle X-ray scattering, and native ion mobility mass spectrometry, we find that oxidation leads to formation of intramolecular dityrosine cross-linkages and a compaction of the alpha-Syn monomer by a factor of root 2. Oxidation-induced compaction is shown to inhibit ordered self-assembly and amyloid formation by steric hindrance, suggesting an important role of mild oxidation in preventing amyloid formation.