Magnetoresistance and Hall effect have been measured at 4.2 K using a hybrid magnet up to 26 T for several crystals of quasi-two-dimensional η-Mo_4O_ that has a charge-density-wave (CDW) induced nested multiple carrier band structure. The magnetoresistance is exceptionally large and the Hall resistivity shows a unique magnetic field dependence. We have confirmed the existence of characteristic hysteresis phenomena in both quantities when a field-sweep range exceeds a threshold value (?10 T). The hysteresis effects can be related with an effective and magnetic field dependent irreversible process for the CDW formation and destruction. We have also found that the hysteresis and quantum oscillations in the magnetotransport are strongly sample-dependent
