A compelling link is emerging between the posttranslational modification O-GlcNAc and protein aggregation. A prime example is alpha-synuclein, which forms toxic aggregates that are associated with neurodegeneration in Parkinson's and related diseases. alpha-Synuclein has been shown to be O-GlcNAcylated at nine different positions in in vivo proteomics experiments from mouse and human tissues. This raises the possibility that O-GlcNAc may alter the aggregation of this protein and could be both an important biological mediator of neurodegeneration and also a therapeutic target. Here, we expand upon our previous research in this area through the chemical synthesis of six site-specifically O-GlcNAcylated variants of alpha-synuclein. We then use a variety of biochemical experiments to show that O-GlcNAc in general inhibits the aggregation of alpha-synuclein but can also alter the structure of alpha-synuclein aggregates in site-specific ways. Additionally, an alpha-synuclein protein bearing three O-GlcNAc modifications can inhibit the aggregation of unmodified protein. Primary cell culture experiments also show that several of the O-GlcNAc sites inhibit the toxicity of extracellular alpha-synuclein fibers that are likely culprits in the spread of Parkinson's disease. We also demonstrate that OGlcNAcylation can inhibit the aggregation of an aggressive mutant of alpha-synuclein, indicating that therapies currently in development that increase this modification might be applied in animal models that rely on this mutant. Finally, we also show that the pan-selective antibody for O-GlcNAc does not generally recognize this modification on alpha-synuclein, potentially explaining why it remains understudied. These results support further development of O-GlcNAcylation tools and therapeutic strategies in neurodegenerative diseases.