<p>Oat endosperm uniquely accumulates oils, differing from other Poaceae crops and oilseed species that primarily store oils in embryos. Different photosynthetic organs’ contribution to grain oil accumulation remains unclear. This study investigated the effects of photosynthesis on grain weight and oil accumulation in the ear, flag leaf, and top second leaf of four oat cultivars with varying oil contents, using shading/excision treatments. Results revealed minor effects of leaf photosynthesis on grain weight and the total fatty acid content. Ear shading reduced grain weight by 16.8–36.5%, and the total fatty acid content per grain by 31.9–47.4%, while decreasing oleic acid levels and increasing linoleic acid levels. These effects were more pronounced in high-oil cultivars than low-oil cultivars, with the proportion of total fatty acid content reductions exceeding the proportion of grain weight losses. Further analysis was conducted on the changes in matter accumulation during the development of the high-oil cultivar BY9 under ear shading. Ear shading had minimal impact during early seed stage but substantially affected mid-to-late stages. Transcriptomic analysis indicated that shading altered the expression patterns of genes related to carbohydrate metabolism, fatty acid synthesis, and triacylglycerol assembly as early as the initial developmental phase. Our findings highlight the critical role of ear photosynthesis in determining both oil production and yield in oats, providing new insights into oil accumulation mechanisms in cereal crops.</p> Graphical abstract <p></p>