Background: Oligohydramnios, defined as a decreased amniotic fluid volume, is an important indicator of fetal well-being and is associated with adverse maternal and perinatal outcomes. Its prevalence varies across populations and is influenced by maternal, fetal, and placental factors. Objectives: To determine the prevalence of oligohydramnios among pregnant women in the third trimester and to identify associated maternal and obstetric factors. Materials and Methods: A hospital-based cross-sectional observational study was conducted in the Department of Obstetrics and Gynecology of a tertiary care hospital over a period of six months (January–June 2025). A total of 250 pregnant women with gestational age ≥28 weeks were included using consecutive sampling. Amniotic fluid index (AFI) was measured using the four-quadrant ultrasonographic technique. Oligohydramnios was defined as AFI ≤5 cm. Relevant demographic and clinical data were collected and analyzed using SPSS version 20. Results: The prevalence of oligohydramnios was found to be 12.8%. The majority of affected women were in the 20–30 years age group and primigravida. Most cases were observed between 32–36 weeks of gestation. Commonly associated conditions included hypertensive disorders (28.1%), postdated pregnancy (21.9%), intrauterine growth restriction (18.7%), and maternal anemia (15.6%). Conclusion: Oligohydramnios is relatively common in the third trimester and is frequently associated with identifiable maternal and obstetric risk factors. Routine antenatal ultrasonographic assessment is essential for early detection and timely management to improve maternal and perinatal outcomes.