Recovery of germanium (Ge) from the fly ash produced as a by-product of the integrated coal gasification combined cycle (IGCC) may be a potential source of this valuable element. Water-based Ge extraction procedures are environmentally friendly but non-selective, and some impurities such as As, Sb, and Ni, are also extracted. The leaching of As, Sb and Ni in water Ge extraction was carried out using oxic conditions in different oxidising atmospheres. Experimental results showed that the oxidative aqueous leaching of IGCC fly ash diminished the leachability of these impurities without altering Ge extraction yields. Dissolved oxygen in aqueous leaching led to the formation of Fe-(oxy)hydroxides structures from the oxidation of Fe-sulphides contained in IGCC fly ash, which diminished the concentration of metals and metalloids in the leachates through co-precipitation, adsorption or ion exchange mechanisms. The oxidation of Fe-sulphides and the formation of Fe-(oxy)hydroxides proved to be the limiting step of the process. The oxidative leaching of As and Sb showed a similar trend, being subsequently removed in the neoformed structures. The removal process was less efficient for Ni. The most selective water extraction for Ge was obtained with pure oxygen at 90 °C and a stirring speed above 750 min−1.