Vibrational absorption of semi‐insulating O‐containing GaAs samples has been investigated under high‐resolution spectroscopy at low temperature and many sharp lines due to localized modes have been observed. Some of them, between 714 and 730 cm−1had already been ascribed to some (Ga,O)‐related center having features common with the vacancy‐oxygen defect in silicon, but a new state related to this center, giving a local mode near 713 cm−1is reported here for the first time. Another new low‐frequency mode at 604 cm−1is also reported, involving probably nitrogen or more likely oxygen bonded to two next‐nearest‐neighbor Ga atoms. The attribution of new modes in the 950–1400 cm−1spectral region to complexes involving oxygen, arsenic, and eventually impurities with 100% isotopic abundance is discussed. The effects of near‐infrared illumination on the observation of the (Ga,O)‐related modes between 713 and 730 cm−1and on a new mode at 983 cm−1have been studied. Tentative explanations based either on metastability or on photo‐induced changes of the charge states of the centers are proposed. These results seem to show that in GaAs, oxygen decorates native centers and that this kind of study could provide a clue to their understanding.