The oscillator strength of different subband excitons in InGaAs/GaAs quantum wells is studied for the first time using Fourier transform reflectance spectroscopy, and is found to take maxima with the well width variation. However, the maximum oscillator strength occurs at a wider well width for excitons with higher quantum numbers. It is shown that the reduced masses of different subband excitons can be accurately determined from the measured oscillator strength.