Vibrational sum frequency generation (VSFG) spectroscopy is used to probe the polymer-silica interface of poly(3-hexylthiophene) (P3HT) organic field-effect transistors (oFETs) in situ during device operation. The VSFG spectra exhibit dramatic changes upon charge accumulation at the buried interface. Proper modeling of the data reveals that the changes in the spectroscopic features are almost exclusively due to changes in the amplitude and relative phase of the nonresonant signal, while the P3HT alkyl CH 3 and CH2 vibrations remain unperturbed. We interpret the spectroscopic data in light of vibrationally resonant, electronically resonant, and electric field dependent enhancements that occur upon oxidative doping of the P3HT, as measured by visible to mid-IR spectroelectrochemistry. Notably, we observe electric field enhancement of the VSFG signals at both positive and negative gate biases despite unipolar current-voltage responses, which we attribute to the trapping of electrons at the dielectric interface.