We investigated factors affecting the accuracy of the oscillometric fixed-ratio blood pressure measurement method. We specifically applied a parametric sensitivity analysis to a mathematical model of oscillometry. We found that changes in arterial stiffness at zero transmural pressure and pulse pressure represent major factors that can potentially cause large error in the method. In particular, our theoretical analysis demonstrates that the fixed-ratio method predicts (1) higher systolic and lower diastolic blood pressures as the artery stiffen over the zero transmural pressure regime and (2) higher systolic and lower diastolic blood pressures as the pulse pressure increases. Further, the impact of pulse pressure on these errors is more obvious as the artery stiffens.