In oxygen‐rich GaAs the optical absorption of substitutional oxygen (OAs) in the near infrared was investigated by measuring its magnetic circular dichroism (MCDA). An MCDA was discovered which originates from a paramagnetic ground state. By investigating its thermal decay kinetics and its dependence on optically induced recharging processes involving EL2 defects and by comparison with analogous local vibrational mode measurements, it could be shown that the MCDA is due to the metastableB′ charge state of the OAscenters. TheB′ state is thus identified as the paramagnetic charge state of the OAscenter. ©1995 American Institute of Physics.