It is critical to understand oxygen (O) ingress behavior in titanium alloys at high temperatures, since it was directly associated with their high-temperature performance. Investigations revealed that the diffusion of O would result in a uniform alpha-case layer beneath the surfaces of titanium alloys. This layer greatly affected the mechanical performance of the alloys. In the present study, a novel O ingress phenomenon in an alpha + beta titanium alloy was observed. After the isothermal treatment in the air, a two-layered region beneath the specimen surfaces was formed: the outer layer (layer A) uniformly grew parallel to the diffusion direction of O; while the inner layer (layer B) precipitated within the first beta grain beneath layer A, arranging in specific orientations. By experimental investigation as well as theoretical analysis, the formation mechanism of the unique layers was understood: with the diffusion of O, an alpha + beta layer (layer A) was uniformly formed beneath the specimen surfaces, while creating a new interface between layer A and the matrix. This interface acted similarly as alpha(GB)( )during the precipitation of alpha(WGB) (layer B). In addition, Burgers orientation relationship was strictly obeyed during the formation of layer B.