This study examined the effects of dietary Mannan-Oligosaccharides (MOS) on growth performance, serum biochemical indices, immune function, and intestinal morphology in fattening Tan sheep. Twenty-eight four-month-old male Tan sheep were assigned to four groups and received a basal diet supplemented with 0%, 1%, 2%, or 3% MOS for 56 days following a 14-day adaptation period.Results indicated that MOS supplementation significantly increased average daily gain (ADG) in a dose-dependent manner (P ≤ 0.05). Serum biochemical analyses showed a linear and quadratic reduction in triglyceride (TG) levels, and a linear and quadratic increase in alkaline phosphatase (ALP) levels (P ≤ 0.05). Total cholesterol (TC) levels decreased linearly, whereas urea nitrogen (UN) levels increased linearly with higher MOS levels (P ≤ 0.05). Mannan-Oligosaccharides also enhanced total antioxidant capacity (T-AOC) and catalase (CAT) activity, with both parameters increasing linearly (P ≤ 0.05). Immune function was significantly improved, demonstrated by elevated levels of immunoglobulin A (IgA), interleukin-6 (IL-6), IL-10, and interferon-gamma (IFN-γ) (P ≤ 0.05). Additionally, 2% and 3% MOS groups showed increased immunoglobulin G (IgG) and tumor necrosis factor-alpha (TNF-α) levels (P ≤ 0.05). Linear increases were noted in serum IgG, IgA, IgM, IL-6, and TNF-α levels, while IL-10 and IFN-γ levels exhibited both linear and quadratic increases (P ≤ 0.05).Intestinal morphology analysis revealed significant improvements in jejunal villus height, crypt depth reduction, and the villus-to-crypt ratio, with these parameters showing both linear and quadratic changes in response to increasing MOS levels (P ≤ 0.05). In conclusion, 2% MOS supplementation is recommended for enhanced growth performance, metabolic health, immune function, and intestinal integrity in fattening Tan sheep.