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Introduction The oral glucose tolerance test (OGTT), though valuable, is underutilized in outpatient settings for the diagnosis of type 2 diabetes mellitus (T2DM). Our study aimed to confirm prediabetes and assess the risk of developing T2DM in individuals with elevated risk, comparing the diagnostic utility of OGTT with fasting plasma glucose (FPG), post-prandial plasma glucose (PPPG), and HbA1C. Methods A hospital-based cross-sectional study was conducted in the Department of General Medicine of a tertiary care hospital in South India, including adults over 18 years with elevated random plasma glucose (RPG) (≥140 mg/dl), FPG (110-125 mg/dl), and HbA1C (5.7-6.4%). After an overnight fast, participants received 75 g of glucose, and venous blood samples were collected at fasting, one hour, and two hours to measure plasma glucose. Results Of the 113 participants, 55.8% (63) were males and 57.5% (65) were overweight or obese (BMI ≥ 23 kg/m²). FPG classified 44.3% (50) as prediabetic and 9.7% (11) as diabetic. While HbA1C categorized 63.7% (72) as prediabetic and 18.6% (21) as diabetic (cutoff >200 mg/dl), the one-hour OGTT categorized 32.7% (37) as prediabetic (cutoff: 155-199 mg/dl) and 44.2% (50) as diabetic. The one-hour OGTT showed greater sensitivity for detecting at-risk individuals. Conclusions The one-hour OGTT, compared to FPG, PPPG, and HbA1C, offers enhanced sensitivity in confirming prediabetes and unmasking T2DM in the prediabetic population. Incorporating the one-hour OGTT into routine screening may improve early diagnosis and intervention, especially in at-risk populations.