An important global cereal used for livestock feed and human nutrition, yet its productivity is increasingly challenged by climate-driven biotic and abiotic stresses. To accelerate the development of dual-purpose, stress-resilient, and high-yielding cultivars for subtropical agroecologies, we assessed the genetic structure and agronomic performance of a 169-line oat association panel. The panel was phenotyped for key vegetative, forage-quality, and grain traits across three subtropical sites in Sub-Saharan Africa over two seasons and genotypes using GBS. SNP-based diversity analyses revealed moderate genetic variation (He = 0.39; PIC = 0.30), and population structure analysis identified two major subgroups with evidence of admixture. Genome-wide association analysis detected 42 significant SNP–trait associations (FDR < 0.05) and 46 candidate genes. Notable candidates included homologs of rice OsAK3 and GSA1, associated with grain size, and a VPS25 homolog implicated in intracellular trafficking and endosperm development. These candidates were located near QTL influencing seed dimensions and thousand-grain weight. By elucidating the genetic basis of yield, quality, and resilience traits, this study provides valuable genomic resources and trait-linked markers to support genomics-assisted breeding of climate-resilient, high-performing oat varieties for subtropical production systems.