  We consider the critical relaxation of the Ising model, the so-called model
A, and study its operator product expansion. Within perturbation theory, we
focus on the operator product expansions of the two-point function and the
response function. At the fixed point, we normalize the coefficients and the
scaling variables so that the result displays universality. The role of the
fluctuation-dissipation theorem is also discussed, and it is shown that it
provides non-perturbative relations among the operator product expansion
coefficients. Finally, the large N limit is considered.
