We analyze the Zeeman effect of the 4/13/2 → 4/15/2 transition of Er3+ in GaAs:Er,O grown by metal-organic vapor-phase epitaxy. The photoluminescence spectrum has been assigned previously to one specific Er-O complex. The dominant optical transition at 1538 nm (6499.5 cm−1), which shows a full width at half maximum of only 0.05 cm−1, has been investigated by high-resolution Zeeman spectroscopy. A highly anisotropic Zeeman pattern is found which indicates the low symmetry of the underlying complex. A detailed analysis shows that the defect has a predominant rhombic symmetry C
,. Additionally, smaller contributions of a crystal field with a monoclinic symmetry C
are found. The results provide further arguments that an ErO2 complex is the responsible center observed.