Boosting the generation of reactive oxygen species by photocatalytic oxygen activation is pivotal, and yet remains a fundamental challenge. Oxime-based metal covalent organic frameworks (MCOFs) are particularly attractive for their redox activity and electronic coupling between metal centers and organic linkers, where linkage chemistry plays a key role in modulating π-conjugation and oxygen activation efficiency. Herein, we demonstrate how linkage engineering unlocks the photocatalytic potential of oxime-based MCOFs. Two nickel-based MCOFs (I-Ni-COF and NQ-Ni-COF) sharing identical building blocks but differing in linkage chemistry—imine versus non-substituted quinoline—were designed and synthesized. The quinoline linkage in NQ-Ni-COF extends π-conjugation, narrowing the bandgap and enhancing visible-light absorption, while also improving charge separation and transfer dynamics. These attributes endow NQ-Ni-COF with superior oxygen activation capability, leading to efficient generation of reactive oxygen species (ROS) and excellent photocatalytic degradation performance of ciprofloxacin (CIP). Specifically, NQ-Ni-COF achieves 92.9% removal of 50 mg L−1 CIP in 60 min under visible light—drastically outperforming I-Ni-COF (24.2%), and exhibits a ninefold enhancement in the degradation rate constant. The material also exhibits excellent stability and recyclability. Mechanistic studies elucidate the role of various ROS and propose degradation pathways.