In this study, we assessed the effect of the length of the efferent limb on weight loss in patients undergoing One Anastomosis Gastric Bypass (OAGB) surgery for morbid obesity. The secondary endpoints evaluated in this study include the nutritional deficiencies that may occur when changing the length of the efferent limb, the resolution of obesity-related co-morbidities, as well as postoperative morbidity and occurrence of complications (e.g., port-site and internal hernias). Methods: This prospective study was conducted on 46 patients with morbid obesity who underwent laparoscopic OAGB. Patients were followed up for one year. Results: The data showed that there is a negative statistical correlation (p <0.05) between the efferent limb length and the excess body weight loss at 3-, 6-, and 12 months post-OAGB. Besides, there was a negative statistical significance (p <0.05) between the percentage of efferent limb length to the total small bowel length (TSBL) and the excess body weight loss at 3-, 6-, and 12 months post-OABG. On the other hand, there were no statistical associations (p >0.05) between the efferent limb length (or the percentage of efferent limb length to total small bowel length) and nutritional deficiencies (albumin, total calcium, and iron) at 12 months post OAGB in this study. Conclusions: In conclusion, measuring the TSBL in each patient undergoing OAGB would be more accurate than the classic estimation given by the anatomy books. The length of the efferent limb is inversely correlated with postoperative excess body weight loss.