We present Cluster observations made during an outbound orbit on 10 December 2000. After exiting the magnetosphere at midlatitude, Cluster spent a long time skimming the magnetopause moving to lower latitude along an orbit approximately in the ZYGSM plane on the dusk flank of the magnetopause. During this time, magnetospheric oxygen with energy ≥10 keV was observed continuously both in the magnetosphere and in the magnetosheath by the Cluster Ion Spectrometry (CIS) plasma experiment. While the oxygen density is roughly constant in the magnetosheath throughout the event, its velocity shows a strong dependence on the magnetosheath magnetic field orientation: low speeds, corresponding to almost isotropic distribution functions, occur for northward magnetic field, and high speeds, corresponding to beam-like distribution function occur for southward magnetic field. Mainly, two different processes have been discussed to explain the energetic particles escaping from the magnetosphere: flow along reconnected magnetospheric and magnetosheath field lines or crossing of the magnetopause when the particle gyroradii are comparable with the magnetopause thickness. The presence of the oxygen population cannot be readily explained in the framework of the reconnection theory. Instead, the observations are successfully reproduced by a model based on magnetopause crossing by finite gyroradius, provided the magnetosheath convection is taken into account together with the magnetosheath magnetic field orientation. Moreover, the presence of quasi-periodic motion of the magnetopause surface with period of approximately 5 min are evidenced by the analysis.