Growth stage alters substrate availability and moisture in oats, potentially driving microbial succession and metabolite formation during ensiling. Oats were harvested at flowering (FS), milk ripening (MS) and wax ripening (DS) stages and vacuum-bag ensiled for 120 d (n = 4 per stage). We measured chemical composition and fermentation products, enumerated key microbes, profiled bacterial communities by 16S rRNA gene amplicon sequencing, and characterised metabolites by untargeted LC–MS. With advancing growth stage, dry matter (DM), neutral detergent fibre (NDF) and acid detergent fibre (ADF) increased, whereas crude protein (CP) decreased. MS silage had the lowest pH (4.63) and the highest CP, whereas FS showed higher lactic acid, but the butyric acid (BA) and NH₃-N/TN were also significantly higher than those at MS and DS (p < 0.001). Community composition shifted from Enterobacter (FS) toward Lactobacillus enrichment (DS), accompanied by stage-specific metabolite signatures (biogenic amines and indoleacetic acid at FS; betaine and purine metabolites at MS; reduced amines and more amino-acid derivatives at DS). Overall, under the conditions of this study, considering fermentation stability, nutrient preservation, microbial diversity, and metabolic pathways, the best balance was achieved during MS.