  The off-equilibrium purely dissipative dynamics (Model A) of the O(N) vector
model is considered at criticality in an $\epsilon = 4- d > 0$ up to
O($\epsilon^2$). The scaling behavior of two-time response and correlation
functions at zero momentum, the associated universal scaling functions, and the
nontrivial limit of the fluctuation-dissipation ratio are determined in the
aging regime.
