Hybrid density functional calculations have been carried out on cluster models of the pristine and oxidized hydrogen-terminated Si(100)-2 x 1 surface to investigate the changes in the surface infrared absorption spectrum upon incorporation of oxygen. Due to a combination of geometric and electronic effects, a dramatic increase in the v(Si-H) band intensity is found for the oxidized surface. An increase in infrared intensity of up to 62% is predicted for multiply oxidized sites. The results presented here may prompt the reinterpretation of infrared studies examining silicon surface chemistry using the v(Si-H) region to probe changes in surface structure and coverage.