Poly(oligoethylene glycol methacrylate) (POEGMA) and zwitterionic polymer brushes have been widely used for constructing biocompatible or antifouling surfaces, and their oxidative stability is very important to the practical application. Herein, POEGMA, poly(sulfobetaine methacrylate) (PSBMA), poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC), and poly(carboxybetaine methacrylate) (PCBMA) were grafted on quartz crystal microbalance (QCM) chips via surface-initiated atom transfer radical polymerization (SI-ATRP). XPS and MS analyses demonstrate that the mass loss of these polymer brushes in oxidative environment is due to the scission of the polymer-anchoring segments. Molecular simulation further illustrates this mass loss mechanism should be always true for those polymer brushes anchored on different substrates. In situ QCM monitoring indicates that, compared with zwitterionic polymethacrylates, POEGMA brushes show the lowest mass loss rate mainly due to their cross-linked structures. This study sheds light on the contradictory reports about the oxidative stability of POEGMA and zwitterionic polymethacrylate brushes up to now, and highlights the important role of the polymer-anchoring segments playing in the oxidative stability of polymer brushes.