Chitosan (CTS), COST (chitosan oligosaccharide, MW ≤ 1000 Da), and COSM (chitosan oligosaccharide, MW ≤ 3000 Da) reduce weight gain, decrease adipose tissue, and increase thermogenesis. Male Sprague Dawley (SD) rats (8 weeks old) were fed a high-fat diet (HFD) for eight weeks to establish an obesity model. These rats received orlistat (7 mg/kg·day), COST (600 mg/kg·day), COSM (600 mg/kg·day), and CTS (600 mg/kg·day) for 8 weeks. The daily food intake (A), weekly body weight (B), and body weight gain (C) of rats were recorded during the experimental period (n = 10 mice per group). The fat content (D) and epididymal fat cells (E) in the treatment group were significantly reduced, and their oral glucose tolerance test (OGTT) was significantly improved (F). The adjacent bar graphs represent the area under the curve (AUC) of OGTT. Increased rectal temperature in the treated group at normal temperature (22 °C) (G) and cold stimulation (4 °C) (H) indicated increased thermogenesis. The red arrow indicates the relatively small volume of fat cells. Data are expressed as the mean ± Standard Deviation (SD), * p < 0.05 and ** p < 0.01 vs. the HF group.