Oat is used for animal feed and human food. It is well adapted to a wide range of environments, but itrngrows well in cool-temperate and tropical African regions, particularly Ethiopia. Ethiopia is one center ofrnorigin for A. abyssinica and A. vaviloviana oat species, but in Ethiopia oat is a minor crop and only a fewrncultivars have been used. Despite the many benefits oat has, the production and productivity of the croprnhave been highly constrained by the inadequacy of improved varieties and narrow genetic base of therngermplasm used in breeding. Aluminium toxicity is a substantial agronomic problem in Ethiopia andrndeveloping Al tolerant oat cultivar is crucial. Therefore, this study was primarily aimed to assess therngenetic diversity, population structure, relation between characters and evaluating the aluminum toxicityrntolerance potential of oat accessions in Ethiopia. A total of 176 accessions from five Avena speciesrnnamely, A. sativa (82), A. fatua (18), A. sterilis (12), A. abyssinica (44), and A. vaviloviana (20) were usedrnfor morpho-agronomic experiment and molecular genetic diversity study. Except for the number ofrnaccessions of A. sativa obtained from USA, the Netherlands, and Australia all the accessions were fromrnEthiopia. The mopho-agronomic experiment was conducted using RCBD design at Debre Brehan andrnHoletta Agricultural Research Centers. Data were collected on 22 qualitative and 12 quantitative traits.rnThe Shannon diversity index was analyzed for qualitative traits. The mean, ANOVA, heritability, geneticrnadvance, principal component analysis, clustering, cluster distance, and association of traits were analyzedrnfor the quantitative trait. The molecular genetic diversity analysis was carried out at ILRI-BecA Hub,rnNairobi, Kenya, using 19 SSR markers with fluorescently labeled forward primers. The analysis of thernallelic pattern and distribution, MANOVA, clustering, PCoA, population structure, and differentiationrnwere analyzed. The hydroponic experiment was conducted at Addis Ababa University to evaluaternaluminum toxicity tolerance of the accessions. The hydroponic experiment was carried out on 150rnaccessions including accessions from, A. sativa (74), A. fatua (16), A. sterilis (5), A. abyssinica (40), andrnA. vaviloviana (15). Regarding morpho-agronomic traits, accession had low to high heritability andrngenotypic advance of mean. The wide range of heritability and genotypic advance of mean were found inrnfive Avena species. Similarly, wide range correlation of grain yield with other traits were observed. ThernAgglomerate hieratical clustering at a phenotypic level among five Avena species revealed three clusters.rnThe first two clusters were predominantly consisted of A. sativa accessions with only a few wild Avenarnaccessions and the third one cluster with two sub-clusters consisted accessions of wild species. Mostrnaccessions of A. abyssinica and A. fatua were clustered together in one group (sub-cluster 1) regardless torntheir geographical origin, and similarly A. sterilis and A. vaviloviana accessions were found together inrnanother group (sub-cluster 2). The distances between clusters of species were different and relatively thernhighest cluster distance was observed between A. sativa and A. abyssinica accessions. Results of analysisrnof molecular variance (AMOVA) showed that 26% variation was between the wild and the cultivatedrnspecies. Calculated FST value between wild and cultivated species was high (FST = 0.080), with moderaterngene flow (Nm = 0.943). In addition, 33% of genetic variation was due to the differences among oatrnspecies. The calculated FST value among oat species was high (FST = 0.325) with a moderate gene flowrn(Nm = 0.519). We found FST value of A. sativa populations and A. abyssinica populations to be 0.051 andrn0.034, respectively. Unweighted pairwise method with arithmetic mean (UPGMA) clustering andrnPrincipal Coordinate analyses (PCoA) revealed that accessions from different collection sites clusteredrntogether. Three groups of clusters were recognized in the UPGMA Dendrogram, the first cluster containedrnA. abyssinica and A. fatua, the second cluster contained A. vaviloviana and A. sterilis, and the third clusterrncontained only A. sativa accessions. Not only the UPGMA clustering but also had the PCoA clusteredrnrevealed the wild and cultivated species grouped separately. At K=2 (optimal gene pool number), tworngene pools were found; one dominated by the cultivated oat and the second by wild oat species.rnPredominantly, A. sativa had one gene pool type, whereas, A. sterilis and A. vaviloviana had an admixturernof the two gene pools. A. abyssinica and A. fatua accessions had the second gene pool type (wild genernpool). The oat species was variable in Al3+ toxicity tolerance to acid soil and the majority of the accessionsrnwere tolerant to aluminium toxicity. On the basis of the species type, ploidy level, and genome type, thatrna comparison was made and it was found more similarity between species of different ploidy levels andrngenome type, for example, A. sterilis and A. vaviloviana and between A. abyssinica and A. fatua thanrnspecies having similar genome and ploidy level. A. sativa had relatively high genetic distance to the fourrnwild Avena species, especially A. abyssinica and A. fatua. Low differentiation among populations mayrnimply significant gene flow through the exchange of planting materials among farmers. The high geneticrndiversity from morpho-agronomic traits within a population would be harnessed for oat improvement.rnThat the majority of the accessions are aluminium toxicity tolerance shows the potential of the oat tornproduce cultivars tolerant to acidic soil. The best top 5% performing accessions for grain yield and otherrnimportant agronomic traits and the best top 5% aluminium toxicity tolerant accessions were identified. Itrnis recommended that the breeder would be consider these for the future improvement of oat crop. It isrnrecommended that future research would be for production stability of identified accessions across a widernrange of environment and their Al toxicity potential in the actual field condition. Thus, it is consideredrnthis study will provide useful information for the breeders and conservationists for proper conservationrnmanagement, and utilization of oat germplasm and improve oat production and productivity to sustainrnfood security.