The complex dielectric function of the orientationally disordered molecular mixed crystal [(CH₃)₃CCH₂OH]_0.7[(CH₃)₂C(CH₂OH)₂]_0.3 (NPA_0.7NPG_0.3) covering 11 decades in frequency (10^-2-10⁹ Hz) is measured over a broad and continuous temperature range (163-333 K) which is close to the orientational glass transition (160.9 K). For the first time, the dynamics of the orientational disordered (OD), the supercooled orientational disordered and the orientational disordered glass is studied without a temperature breakdown as usually occurs due to the catastrophe of the crystallization. The orientational glass transition in the OD phase exhibits the essential features of the structural glass transition, a non-Arrhenius behaviour for the temperature variation of the relaxation time and a broadening of the width of the relaxation with decreasing temperature. The temperature dependence of the relaxation time is analysed by the empirical Vogel-Fulcher-Tammann law as well as the more recent mode-coupling theory. The suggested increase of the ratio between the crossover temperature T_c and glass transition temperature T_g with decrease of fragility of the system is in accordance with the present observation for this mixed crystal (T_c/T_g≥1.45).