Oats (Avena sativa L.) are rich in nutrients and bioactive compounds, serving as a roughage source for ruminants. This study investigated the impact of lactic acid bacteria (LAB), cellulase (M), and their combinations (LM) on the fermentation quality and metabolic compounds of oat silage. Results demonstrated that all additive treatments significantly increased lactic acid content compared to the control group (P < 0.05), with the lactic acid bacteria treatment group exhibiting the lowest pH value (P < 0.05). Analysis of antioxidant activity and metabolites in oat silage over 60 days revealed 374 differential metabolites with 113 up-regulated and 261 down-regulated, and all treatment groups showing higher antioxidant activity than raw oat materials (P < 0.05). Although no significant differences in antioxidant activity were observed among the various treatment groups in this experiment, notable changes in metabolic pathways were identified. Furthermore, two metabolites (carboxylic acids and derivatives and benzene and substituted derivatives) were identified through non-targeted metabolomics technology, both of which are strongly associated with the antioxidant activity of oat silage. This finding provides a theoretical basis for the efficient use of oat silage in animal husbandry.