Alpha-Synuclein (alpha-Syn) aggregates represent the major component of Lewy bodies (LBs), a pathologic hallmark of Parkinson's disease (PD). Current reports have assessed the toxicity of oligomeric alpha-Syn (o-alpha-Syn) mostly in vitro after the incubation with PBS, which leaves o-alpha-Syn non-phosphorylated and does not reflect actual physiologic conditions in PD patients. The present study aimed to assess the pathogenic role of o-alpha-Syn while addressing the above issues using o-alpha-Syn incubated with PD plasma. Several alpha-Syn oligomer types were prepared by incubating recombinant human alpha-Syn with phosphate-buffered saline (PBS), and plasma samples from normal controls (NS) and PD patients. O-alpha-Syn incubated with PD plasma (o-alpha-Syn-PD), moderately or highly phosphorylated at serine 129, induced cell death more substantially compared with the PBS and NS groups. PD plasma exhibited reduced PP2A activity and ceramide levels, promoting the phosphorylation of o-alpha-Syn. In agreement, ceramide addition alleviated o-alpha-Syn-PD cytotoxicity. In vivo, o-alpha-Syn-PD significantly reduced dopaminergic neurons in the substantia nigra and could be transferred to the cortex, hippocampus, and other parts of the brain. Mice administered o-alpha-Syn-PD exhibited significant PD-like dyskinesia changes in a short period of time. Finally, o-alpha-Syn-PD injection was associated with decreased GCase and PP2A activities in the mouse brain. The above findings provide novel insights into the effect of o-alpha-Syn on neurodegeneration in PD and dementia with LBs.