In power system stability classification, rotor angle oscillation and frequency oscillation belong to dynamic rotor angle stability and small-disturbance frequency stability respectively. In existing studies, coherent oscillation of generator speeds is regarded as the key feature that distinguishes frequency oscillation from rotor angle oscillation, but there is a lack of in-depth investigation. The characteristics and variations of oscillation modes in a typical two-area system are analyzed in the paper. When the electrical connection between the two areas becomes very weak, the phase and amplitude differences between the speeds of the two generator groups in the frequency oscillation mode become notable, and they are no longer coherent. Moreover, as the electrical connection between the two areas becomes weak, the inter-area rotor angle oscillation mode converts into the frequency oscillation mode of one area, and the original global frequency oscillation mode converts into the frequency oscillation mode of the other area. This corresponds to that the original interconnected synchronous system turns into two non-interconnected synchronous systems. Therefore, strictly speaking, generator speed coherency is not the essential distinction between frequency oscillation mode and rotor angle oscillation mode. However, in the range of actual power grid parameters, the distinction is still valid and can be used in engineering. The results are helpful to understand the difference and relationship between frequency oscillation mode and rotor angle oscillation mode.