  We present a theory of organic magnetoresistance (OMR) based on the quenching
of the quantum correlation between the carrier's spin and its local environment
when the incoherent hopping takes place. We prove that this process contributes
a spin-dependent prefactor to the attempt-to-escape frequency in the hopping
rate, with its value modulated by the magnetic field. The resulting OMR
exhibits a positive Lorentzian saturation component and a negative small-field
component, which are independent of model parameters. These behaviors, with
their isotope effects, are in good agreement with experimental results.
