Nonlinear distortions caused by high power amplifiers (HPA) are considered as the key impairment in multicarrier techniques based communication systems. However, there are few past investigations addressing their effect on the performance of filter bank multicarrier (FBMC) based systems. In this letter, we derive a closed-form expression for the BER of Gray-coded M-ary (QAM or OQAM) based OFDM running under frequency-flat fading Rayleigh channel which is valid for any memoryless nonlinearity model and can be evaluated without numerical integration method.