  We present the numerical solution of the leading order QCD evolution equation
for the orbital angular momentum distributions of quarks and gluons and discuss
its implications for the nucleon spin sum rule. We observe that at small-$x$,
the gluon helicity and orbital angular momentum distributions are roughly of
the same magnitude but with opposite signs, indicating a significant
cancellation between them. A similar cancellation occurs also in the quark
sector. We explain analytically the reason for this cancellation.
