In order to study the influence of pre-existing dislocations on the pop-in phenomenon in magnesium oxide, an original experimental protocol, based on low-rate cleavage, has been developed to introduce a controlled dislocation density in MgO single crystals. Nanoindentation tests have been performed using a spherical indenter and the pop-in load has been measured as a function of the pre-existing dislocation density. The dislocation structures have been then characterized individually by chemical etching and atomic force microscopy observations. A double etching method has allowed distinguishing the pre-existing dislocation behaviour in the indenter stress field and the dislocation nucleated below the indenter during the pop-in. These experiments show that the pre-existing dislocations lower the pop-in load and promote the dislocation nucleation below the contact area. However, the analysis of the dislocation structures nucleated during the pop-in shows no direct relation between the pre-existing dislocations and those nucleated during the pop-in.