OH* is one of the major excited radicals in flame. The chemiluminescence information is generally applied in combustion diagnostics to indicate the flame structure, strain rate, equivalence ratio, heat release rate, etc. In this paper, the numerical study on OH* radicals was conducted in the laminar methane/oxygen co-flowing jet diffusion flames. The detailed GRI 3.0 mechanism combined with OH* radicals reaction mechanism was used in the numerical model. Based on the two-dimensional OH* distributions, the pathways of OH* formation in different area were analyzed. The effects of oxygen-fuel equivalence ratio and coaxial nozzle structure on OH* intensity and distribution were also discussed. The simulation results were consistent with the experiment results, indicating that the numerical model can effectively describe the two-dimensional OH* distributions. The results show that there are two different types of OH* distribution areas, and the OH* radicals in these two areas are formed respectively through the reactions CH+O2=OH*+CO and H+O+M=OH*+M. The flame structure can be indicated according to the distribution area of OH* radicals. With increasing the oxygen-fuel equivalence ratio, the distribution area of OH* radicals gradually expands to the downstream of flame. The distinctions of OH* distribution can be used to characterize the combustion condition. If OH* radicals only distribute over the upstream of the flame and appear discrete in shape, the flame is under the oxygen-deficient combustion. If OH* radicals distribution appears as a ring shape, the flame is under the oxygen-enriched combustion. Under the same oxygen flow rate, the size of annular channel has a significant impact on the mole fraction of OH*. Reducing the size of annular channel can enhance the mixture of fuel and oxygen as well as improve the radiation intensity of the OH* chemiluminescence, which makes the flame diagnosis more convenient.