In order to decrease their environmental impact, turbo-engine manufacturers tend to increase the span of fan blades while maintaining a slender profile. This design leads to more pronounced geometrical nonlinear effects. Computing the frequency response function of such structures is complicated due to the size of their associated finite element model. Classical substructuring approaches are no longer efficient to reduce the size of the problem as all the nodes of the system must be kept since they experience nonlinear behaviors. Different reduction methodologies have been defined in the past decades to tackle such nonlinear systems. Among these strategies, the direct normal form (DNF) extends the theory of normal form to finite element models. This methodology is here applied to a single blade model. Based on the assumption of a fairly rigid disk and the cyclic symmetric properties, a full cyclic symmetric reduced-order model is computed. In this work, this methodology is extended to account for random mistuning. Such a strategy allows to perform, for instance, fast parametric studies. This paper studies the sensitivity of the random mistuning on a nonlinear open rotor system in order to help turbo-engineers in their design phase. Three ranges of the excitation level are studied. At a low level of excitation, the system is close to the linear case. For higher forcing amplitude, a high amplification factor (AF) due to the merge of an isolated branch is observed, which is detrimental for the structure. For the last range (containing the highest forcing amplitudes), the nonlinearities are highly activated, and low values of the amplification factor are obtained due to the spread of the vibrational energy over the frequency range.