Recently, it has been shown that water, confined in a plasma polymer subsurface chemical gradient, nanometers below the surface, significantly reduced the amount of adsorbed protein bovine serum albumin (BSA). Relating to this effect, we proposed the hypothesis that oriented water molecules within the subsurface gradient generate a long-range dipolar field, which interacts with dipolar proteins such as BSA near the surface region. This study extends the above used in situ multistep plasma deposition process to introduce plasma oxidation modifications of the subsurface architecture with the aim to further control the effect on protein adsorption. Neutron reflectivity measurements reveal that the oxidation time increases the amount of matrix-confined water. There is, however, an optimal oxidation time to obtain minimal protein adsorption, which suggests that a minimal distance between confined water molecules plays an important role. Altogether we can extend the range of controlling the adsorbed protein mass by the introduction of this additional plasma oxidation step.