We provide a uniformly-positive point-wise lower bound for the two-point function of the classical spin O(N ) model on the torus of Z d , d ≥ 3, when N ∈ N >0 and the inverse temperature β is large enough. This is a new result when N > 2 and extends the classical result of Fröhlich, Simon and Spencer (1976). Our bound follows from a new site-monotonicity property of the two-point function which is of independent interest and holds not only for the spin O(N ) model with arbitrary N ∈ N >0 , but for a wide class of systems of interacting random walks and loops, including the loop O(N ) model, random lattice permutations, the dimer model, the double-dimer model, and the loop representation of the classical spin O(N ) model. *