Berkovich nanoindentations have been performed at ambient temperature with a new apparatus on (001) cleaved surfaces of single-crystal Magnesium Oxide (MgO). Rosette slip lines pattern has been observed at a nanometric scale by Atomic Force Microscope. Moreover, the load-displacement curves present particular features such as pop-in, which has been linked to the AFM surface topography observations. Finally, we show that the combination of a new nanoindentation apparatus and an Atomic Force Microscope allows to study the very early stages of the plastic deformation during a nanoindentation in terms of individual dislocations.