This study investigated the influence of high levels of dissolved oxygen (DO) on the matured aerobic granular sludge (AGS) treating high-strength ammonium wastewater. AGS was cultivated and matured through a SBR after long-term treatment of synthetic influents having the same characteristics (COD = 1250 ± 70, TN = 165 ± 36, and TP = 54 ± 11 mg/L) under limited DO (≤ 2 mg/L). Two identical SBRs were operated for 208 days. In stage I (77 days), DO was 5 - 6 mg/L, while in stage II (days 78 - 146) DO jumped to 7.5 mg/L, and finally, in stage III, DO was ≥ 8.5 mg/L. Initial results indicated that SBRs achieved simultaneous COD/NH4-N/PO4−3-P removal efficiencies of 94/89/91%. Ammonium oxidation rate of 15.4 ± 1.4 mg-NH4/g-MLVSS·h, and simultaneous nitrification–denitrification (SND) has been recorded. Phosphorus release/uptake rates of 58/14 mg-P/g-MLVSS·h, respectively, and efficient physical properties (size ≥ 2.7 mm and sludge volume indexes were ≤ 31 mL/g) were detected. On day 75, ammonium oxidation rate jumped to 16.1 ± 1.1 mg-NH4/g-MLVSS·h and PO4−3-P effluent concentrations were reduced to 1.3 mg/L. However, results obtained during the following stages did not show any significant development in reactors’ performance. Effluent ammonium and nitrate concentrations on day 146 were 8.2 and 1.8 mg/L, while on day 208 they were 7.7 and 3.8 mg/L, respectively. PO4−3-P was increased to 1.4 mg/L on day 208, and deterioration of AGS accompanied with reduction in the secretions of extracellular polymeric substances occurred after applying DO ≥ 8.5 mg/L.