Purpose <p>One-anastomosis gastric bypass (OAGB) is an established bariatric procedure for managing obesity and related metabolic conditions. This study evaluated the impact of OAGB on weight loss, metabolic markers, and nutritional outcomes over one year, with specific focus on the percentage of small intestine bypassed.</p> Methods <p>In this prospective study, 42 patients (aged 29–65 years; BMI &gt; 35&#xa0;kg/m²) underwent OAGB with intraoperative measurement of total small bowel length to calculate bypass percentage. Patients were followed at six months and one year. Clinical and biochemical parameters—including BMI, HbA1c, lipid profile, transferrin, malnutrition, and steatorrhea—were assessed. Generalized estimating equations and Bonferroni-adjusted pairwise comparisons were used (SPSS v24; significance level &lt; 0.05).</p> Results <p>The mean bypass percentage was 34.9 ± 4.7%. Mean BMI decreased significantly from 46.5 ± 7.9&#xa0;kg/m² preoperatively to 29.0 ± 5.3&#xa0;kg/m² at one year (<i>P</i> &lt; 0.001). Higher bypass percentages were associated with greater BMI reduction. HbA1c decreased significantly (<i>P</i> &lt; 0.001) but was not related to bypass percentage. HDL increased while LDL and triglycerides declined significantly over time (<i>P</i> &lt; 0.05). Transferrin levels remained unchanged. The incidence of malnutrition increased (<i>P</i> &lt; 0.001) and steatorrhea decreased (<i>P</i> &lt; 0.05) at one year; neither was significantly associated with bypass percentage.</p> Conclusion <p>OAGB led to significant weight loss and metabolic improvements over one-year follow-up. Greater bypass percentage correlated with greater BMI reduction but did not affect glycemic or nutritional outcomes. Ongoing monitoring is recommended to manage potential nutritional risks.</p>