Phospholipid vesicles are commonly used to get insights into the mechanism by which oligomers of amyloidogenic proteins damage membranes. Oligomers of the protein α-synuclein (αS) are thought to create pores in phospholipid vesicles containing a high amount of anionic phospholipids but fail to damage vesicle membranes at low surface charge densities. The current understanding of how αS oligomers damage the membranes is thus incomplete. This incomplete understanding may, in part, result from the choice of model membrane systems. The use of free-standing membranes such as vesicles may interfere with the unraveling of some damage mechanisms because the line tension at the edge of a membrane defect or pore ensures defect closure. Here, we have used supported lipid bilayers (SLBs) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPC/POPS) to study the membrane damage caused by αS oligomers. Although αS oligomers were not able to initiate the disruption of POPC/POPS vesicles or intact SLBs, oligomers did stabilize and enlarge pre-existing SLB defects. The increased exposure of lipid acyl chains at the edges of defects very likely facilitates membrane–oligomer interactions, resulting in the growth of fractal domains devoid of lipids. Concomitant with the appearance of the fractal membrane damage patterns, lipids appear in solution, directly implicating αS oligomers in the observed lipid extraction. The growth of the membrane damage patterns is not limited by the binding of lipids to the oligomer. The analysis of the shape and growth of the lipid-free domains suggests the involvement of an oligomer-dependent diffusion-limited extraction mechanism. The observed αS oligomer-induced propagation of membrane defects offers new insights into the mechanisms by which αS oligomers can contribute to the loss in membrane integrity.