Dissolved oxygen concentration in water is a crucial parameter to assess the condition or evolution of an aquatic ecosystem health. This determination can be performed using an electrochemical sensor or the reference Winkler method that allows a more reliable measurement of this parameter. The comparison of dissolved oxygen values determined on two occasions or two samples requires calculating the measurement uncertainty. This uncertainty is also relevant to understand if the determination has adequately low uncertainty. This work describes the detailed assessment of dissolved oxygen determinations' performance aiming at the 'bottom-up' quantification and optimisation of the measurement uncertainty. The visual end-point detection's uncertainty was estimated by the difference between observed measurement precision and combined models of all precision components except the end-point detection. A user-friendly MS-Excel spreadsheet that allows applying the developed uncertainty evaluation procedure is made available as Supplementary Material. The determination of dissolved oxygen from analytical portions not lower than 50 mL is fit for environmental monitoring. It allows measurements between 0.3 mg L⁻¹ and 14.6 mg L⁻¹ with an expanded uncertainty between 0.36 mg L⁻¹ and 0.74 mg L⁻¹ for a 95% confidence level. This uncertainty allows differentiating dissolved oxygen values between 0.51 mg L⁻¹ and 1.0 mg L⁻¹ with less than a 5% probability of being wrongly assumed a relevant difference. The described uncertainty evaluation strategy can also be used in other titrimetric determinations.