Herein, we account for a modified glassy carbon electrode with a copper-1-amino-2-naphthol (AN–Cu²⁺) complex immobilized on a carboxyl-functionalized multiwalled carbon nanotube (f-MWCNT) (GCE/f-MWCNT@AN–Cu²⁺). It is employed for the electrochemical detection of dissolved oxygen (DO) in 0.1 mol L⁻¹ phosphate buffer solution (PBS). The oxygen reduction reaction was exclusively catalyzed by the AN coated f-MWCNT. Cu²⁺ was merely used to immobilize AN on the f-MWCNT to enhance the stability of the electrode. To prove the interaction between O₂ and AN, a dilute solution of AN in pH 7 PBS was monitored by purging N₂ and O₂, and an intense turbidity was observed, with O₂ indicating the adduct formation between AN and O₂. The prepared sensor exhibits an excellent tolerance to interferents which are found in water samples along with oxygen. The Koutecky-Lévich analysis confirms the four-electron reduction of O₂. Moreover, reproducibility, stability, the limit of detection (LOD), and sensitivity of the modified electrode were found to show the capability of the proposed sensor for monitoring the DO content.