Dissolved Oxygen (DO) plays a key role in reactive processes and microbial dynamics in the critical zone. While the general view is that oxygen is rapidly depleted in soils and that deeper compartments are anoxic, recent observations showed that fractures can provide rapid pathways for deep oxygen penetration, triggering unexpected biogeochemical processes. As it is transported in the subsurface, DO reacts with electron donors, such as $Fe^{2+}$ coming from mineral dissolution, hence influencing rock-weathering. Yet, little is known about the factors controlling the spatial heterogeneity and distribution of oxygen with depth. Here we present analytical expressions describing the coupled evolution of DO and $Fe^{2+}$ as a function of fluid travel time in crystalline rocks. Our model, validated with reactive transport simulations, predicts a linear decay of DO with time, followed by a rapid non-linear increase of $Fe^{2+}$ concentrations up to an equilibrium state. Relative effects of the reducing capacity of the bedrock and of transport velocity are quantified through a Damkohler number, capturing key hydrological and geological controls of $Fe^{2+}$ and DO distributions in the subsurface. This framework is used to investigate contrasted DO and $Fe^{2+}$ concentrations observed in two crystalline catchments. These differences are explained by the Damkohler number: one system is reaction-limited while the second is transport-limited. We show that hydrological and geological drivers can be discriminated by analyzing both O$_2$ and $Fe^{2+}$. These findings provide a new conceptual framework to understand and predict the evolution of DO in the subsurface, a key element in the critical zone.