Using next-generation DNA sequencing, it was possible to clarify the genetic relationships of Avena species and deduce the likely pathway from which hexaploid oat was formed by sequential polyploidization events. A sequence-based diversity study was conducted on a representative sample of accessions from species in the genus Avena using genotyping-by-sequencing technology. The results show that all Avena taxa can be assigned to one of four major genetic clusters: Cluster 1 = all hexaploids including cultivated oat, Cluster 2 = AC genome tetraploids, Cluster 3 = C genome diploids, Cluster 4 = A genome diploid and tetraploids. No evidence was found for the existence of discrete B or D genomes. Through a series of experiments involving the creation of in silico polyploids, it was possible to deduce that hexaploid oat likely formed by the fusion of an ancestral diploid species from Cluster 3 (A. clauda, A. eriantha) with an ancestral diploid species from Cluster 4D (A. longiglumis, A. canariensis, A. wiestii) to create the ancestral tetraploid from Cluster 2 (A. magna, A. murphyi, A. insularis). Subsequently, that ancestral tetraploid fused again with another ancestral diploid from Cluster 4D to create hexaploid oat. Based on the geographic distribution of these species, it is hypothesized that both the tetraploidization and hexaploidization events may have occurred in the region of northwest Africa, followed by radiation of hexaploid oat to its current worldwide distribution. The results from this study shed light not only on the origins of this important grain crop, but also have implications for germplasm collection and utilization in oat breeding.