One of the main research topics related to Alzheimer's disease is the aggregation of the amyloid-beta peptide, which was shown to follow different pathways for the two major alloforms of the peptide, A beta 40 and the more toxic A beta 42. Experimental studies emphasized that oligomers of specific sizes appear in the early aggregation process in different quantities and might be the key toxic agents for each of the two alloforms. We use transition networks derived from all-atom molecular dynamics simulations to show that the oligomers leading to the type of oligomer distributions observed in experiments originate from compact conformations. Extended oligomers, on the other hand, contribute more to the production of larger aggregates thus driving the aggregation process. We further demonstrate that differences. in the aggregation pathways of the two A beta alloforms occur as early as during the dimer stage. The higher solvent-exposure of hydrophobic residues in A beta 42 oligomers contributes to the different aggregation pathways of both alloforms and also to the increased cytotoxicity of A beta 42.