The characteristics, the formations and loss mechanisms of different particles of hollow cathode discharge in oxygen at 266 Pa are investigated by using the fluid model. The model contains 11 kinds of particles and 48 reactions. Under this simulation condition, the negative glow regions corresponding to the surrounding cathodes overlap. The results show that there is a strong hollow cathode effect. The density distributions of different charged and active particles are calculated. The charged particle density is located mainly in the central region of the discharge cell. Electrons and O^– are the main ingredients of negative charges in the discharge system, and their density peaks are 5.0 × 10¹¹ cm^–3 and 1.6 × 10¹¹ cm^–3, respectively and \rmO_2^+ is a main composition of positive charge in the discharge system with a peak density of 6.5 × 10¹¹ cm^–3. Abundant active oxygen particles exist in the discharge system, and their density is much higher than those of other charged particles. According to the densities of active particles, their magnitudes are ranked in the small-to-large order as O, O₂(a¹Δ_g), O(¹D) and O₃. Furthermore, the generation and consumption mechanism of electrons, O^– and \rmO_2^+ are calculated in detail, and the generation and consumption paths of different active oxygen particles are also given. The results show that there is a complex coupling process among these particles. Each reaction generates a certain number of particles and consumes other particles at the same time, resulting in a dynamic balance among these particles.