The aggregation of α-synuclein (αSN) and increased oxidative stress leading to lipid peroxidation are pathological characteristics of Parkinson's disease (PD). Here, we report that aggregation of αSN in the presence of lipid peroxidation products 4-hydroxy-2-nonenal (HNE) and 4-oxo-2-nonenal (ONE) increases the stability and the yield of αSN oligomers (αSO). Further, we show that ONE is more efficient than HNE at inducing αSO. In addition, we demonstrate that the two αSO differ in both size and shape. ONE-αSO are smaller in size than HNE-αSO, except when they are formed at a high molar excess of aldehyde. In both monomeric and oligomeric αSN, His50 is the main target of HNE modification, and HNE-induced oligomerization is severely retarded in the mutant His50Ala αSN. In contrast, ONE-induced aggregation of His50Ala αSN occurs readily, demonstrating the different pathways for inducing αSN aggregation by HNE and ONE. Our results show different morphologies of the HNE-treated and ONE-treated αSO and different roles of His50 in their modification of αSN, but we also observe structural similarities between these αSO and the non-treated αSO, e.g., flexible C-terminus, a folded core composed of the N-terminal and NAC region. Furthermore, HNE-αSO show a similar deuterium uptake as a previously characterized oligomer formed by non-treated αSO, suggesting that the backbone conformational dynamics of their folded cores resemble one another.