Predissociation of superexcited states of OCS is studied by two-dimensional photoelectron spectroscopy using synchrotron radiation in the photon energy range of 15-16.5 eV. A two-dimensional photoelectron spectrum exhibits two kinds of characteristic patterns both of which are ascribed to autoionization of sulfur atoms. This superexcited atom S* is produced by predissociation of a Rydberg state OCS*(R-B) converging to OCS+ ((B) over tilde (2)Sigma(+)). The pattern of the first kind results from predissociation processes in which the effective principal quantum number n of the Rydberg electron is almost conserved. This suggests that the Rydberg electron behaves as a spectator because of its negligibly weak interaction with the ion core (spectator predissociation). On the contrary, n of S* does not accord with that of OCS*(R-B) in the pattern of the second kind, indicating that the Rydberg electron participates directly in the electron exchange mechanism controlling conversion from OCS*(R-B) to a predissociating state (participant predissociation). With increasing n, OCS*(R-B) decays more preferentially by the spectator than by the participant predissociation. The spectator predissociation of OCS*(R-B) proceeds through a two-step conversion which involves Rydberg states converging to OCS+ ((A) over tilde (2)Pi and (X) over tilde (2)Pi) and a dissociative multiple-electron-excited satellite state OCS*(SAT) asymptotically correlating with S* + CO((X) over tilde (1)Sigma(+)). In contrast, the participant predissociation may be accounted for by a direct conversion from OCS*(R-B) to OCS*(SAT). The quantum yields are estimated to be 0.06 and 0.02 for the spectator and participant predissociation, respectively, at the incident photon energy of 15.95 eV where OCS*(R-B) states with n similar to 12 lie. A simulation is performed to reproduce the partial cross section curve for the spectator predissociation by using a model in which the decay rates for the participant and spectator predissociation are assumed to be proportional to n(-3) and n(0), respectively. The simulated and experimental cross section curves are in good agreement with each other in the photon energy range of 15.8-16.04 eV.