  We predict that singly-occupied carrier traps, produced by electrical stress
or irradiation within organic semiconductors, can cause spin blockades and the
large room-temperature magnetoresistance known as organic magnetoresistance.
The blockade occurs because many singly-occupied traps can only become doubly
occupied in a spin-singlet configuration. Magnetic-field effects on spin mixing
during transport dramatically modify the effects of this blockade and produce
magnetoresistance.We calculate the quantitative effects of these traps on
organic magnetoresistance from percolation theory and find a dramatic nonlinear
dependence of the saturated magnetoresistance on trap density, leading to
values $\sim$ 20%, within the theory's range of validity.
