Key message <p>Identification of promising loci associated with biomass yield traits and utilizing them in breeding programs can potentially enhance genetic improvement of oat biomass yield for forage purposes.</p> Abstract <p>Biomass yield is a complex trait influenced by several loci with small effects. Leveraging genomic tools for oat biomass yield improvement has the potential to accelerate breeding efforts to improve livestock production systems and increase oat production per unit area. We utilized genome-wide association study (GWAS) to identify quantitative trait loci (QTL) and genomic regions associated with biomass yield traits and incorporated the GWAS-derived major-effect QTLs as fixed effects in genomic prediction models. A panel consisting of 420 oat lines was phenotyped for five biomass yield traits in three growing seasons, 2022–2025, and biomass was harvested three times during each growing season. Fifty-eight unique significant loci distributed over 18 oat chromosomes were identified to be associated with biomass yield and related traits, out of which only 16 were major QTLs explaining phenotypic variance &gt; 10%. Four of the QTLs were stable and showed pleiotropic effects. Particularly, the pleiotropic and stable locus Schr4D_320165235 was linked to important candidate genes which are crucial for lignin deposition, improved root system, and enhanced photosynthesis. Stacking of favorable alleles for plant height, ground cover, and growth habit resulted in corresponding significant increase in phenotypic performance for the traits. Incorporating major-effect QTLs as fixed effects did not result in significant improvement in the predictive ability of the traits. The detection of QTLs and novel candidate genes, as well as integration of GWAS-assisted genomic prediction for biomass yield traits, could potentially be useful for enhancing the genetic improvement of oat forage yield traits.</p>