Abstract. We consider operators T : M 0 → Z and T : M → Z, where Z is a Banach space and (M 0 , M ) is a pair of Banach spaces belonging to a general construction in which M is defined by a "big-O" condition and M 0 is given by the corresponding "little-o" condition. Prototype examples of such spaces M are given by ℓ ∞ , weighted spaces of functions or their derivatives, bounded mean oscillation, Lipschitz-Hölder spaces, and many others. The main result characterizes the weakly compact operators T in terms of a certain norm naturally attached to M , weaker than the M -norm, and shows that weakly compact operators T : M 0 → Z are already quite close to being completely continuous. Further, we develop a method to extract c 0 -subsequences from sequences in M 0 . Applications are given to the characterizations of the weakly compact composition and Volterra-type integral operators on weighted spaces of analytic functions, BM OA, V M OA, and the Bloch space.