Bubble formation caused by oxygen supersaturation can influence gas–liquid separation and heat exchange in polymer-electrolyte-membrane (PEM) electrolyser loops, yet the rate at which the dissolved-oxygen level relaxes toward equilibrium in turbulent pipe flow is poorly quantified. This study measures that relaxation in a dedicated test facility. Oxygen-saturated water (170 mg/l, 3.86 bar, 18 °C) was depressurised to 1 bar, generating a saturation ratio of 3.86. The liquid then flowed through three PVC pipes of different lengths (inner diameter = 20 mm) at seven volumetric flow rates corresponding to Reynolds numbers between 6 500 and 27 000. Outlet concentrations were obtained with a self-developed in-situ sampling technique whose combined uncertainty, propagated from the primary measurements, is ±0.06 in the dimensionless degree of desorption Θ_des, excluding potential systematic bias from the sampling process. Θ_des rose with contact length—0.32 (short), 0.40 (medium) and 0.48 (long)—but was independent of flow velocity, showing that the residence-time reduction at higher velocities is offset by a proportional increase in the mass-transfer coefficient. Even after the long configuration (up to 35 s residence) roughly half of the initial supersaturation remained, confirming strong kinetic limitations. A previously published axial cell model reproduced the data when the turbulent mass-transfer coefficient was calculated with the Kress–Keyes correlation; alternative correlations (Lamont–Scott, Avdeev) over-predicted desorption. The results provide benchmark data for oxygen release in a turbulent water flow at moderate supersaturation.