This paper investigates the joint number of subcarriers and power optimization in Multi user Orthogonal Frequency Division Multiplexing based Differential Chaotic Shift Keying systems with noise reduction. We first find a closed-form expression for the optimal number of references in MU OFDM DCSK systems, while existing works only consider the numerical simulations. Moreover, this paper is the first to consider jointly subcarrier and power allocation in the MU OFDM DCSK system by defining a fractional problem. The BER expression of the MU OFDM DCSK system as an objective function is nonconvex and complex, which makes the analytical solution so difficult. We make it tractable by converting the fractional problem into a subtractive form, which leads to a near optimal solution. Simulation results show that our proposed approach provides better BER performance than the existing plans.