To determine the effects of proanthocyanidins on the growth, antioxidant capability, immunity, intestinal morphology, intestinal microbiota, and liver health of Channa argus (10.12 ± 0.12 g), four experimental diets with various oligomeric proanthocyanidin (OPC) concentrations (0, 200, 400, and 800 mg/kg) were fed to triplicate groups for 8 weeks. The final body weight (FBW), weight gain (WG), feeding rate (FER), protein efficiency ratio (PER), and specific growth rate (SGR) were significantly improved in the 400 and 800 mg/kg OPC-supplemented diets (P < 0.05). The superoxide dismutase (SOD), glutathione-S-transferase (GST), glutathione (GSH), catalase (CAT), and glutathione peroxidase (GSH-Px) activities in the liver and intestines were increased significantly, whereas the malondialdehyde (MDA) content exhibited the opposite pattern (P < 0.05). The GST, Cu/Zn superoxide dismutase (Cu/Zn SOD), nuclear factor erythroid 2-related factor 2 (Nrf2), NAD(P)H quinone dehydrogenase 1 (NQO-1), and heme oxygenase 1 (HO-1) mRNA expression levels in the intestines and liver were significantly increased, whereas kelch-like ECH-associated protein 1 (Keap1) expression showed the opposite pattern (P < 0.05). Significant increases were noted in the serum lysozyme (LYS), immunoglobulin M (IgM), complement 3 (C3), and complement 4 (C4) levels, and significant decreases were noted in the serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bile acid (TBA), and total cholesterol (TC) levels (P < 0.05). The interferon gamma (IFN-γ), interleukin-8 (IL-8), interleukin-1 beta (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) mRNA expression levels in the liver and intestines were significantly decreased, whereas the expression of interleukin-10 (IL-10) showed the opposite trend (P < 0.05). Compared with the control diet, the 400 and 800 mg/kg OPC-supplemented diets markedly improved liver and intestinal morphology. With increasing OPC concentration, the relative abundance of the beneficial bacteria, namely, Firmicutes increased, whereas the opposite trend was observed for the potential pathogenic bacteria, namely, Proteobacteria, Actinobacteria, Chlamydiae, Bacteroidetes, Cyanobacteria, and Acidobacteria. Therefore, dietary OPC supplementation facilitates growth performance and enhances antioxidant capability and immunity, as well as improves liver health, intestinal morphology, and microbiota composition of C. argus, of which the optimal level is 400 mg/kg.