High contents of Al+ and O+ ions were implanted sequentially into high purity Fe–10%Cr alloy thin foils at room temperature. The as-implanted foils were then studied by transmission electron microscopy (TEM) using the conventional TEM, energy dispersive X-ray (EDX), energy-filtered TEM (EFTEM) and high-resolution TEM (HRTEM) methods. In contrast to the conventional precipitate ensemble synthesis by implantation/annealing, the synthesis of clusters took place already at the implantation stage without requiring any subsequent thermal annealing in our case. The observed precipitates with diameters in the range of 3–25 nm were enriched in Al and O. The crystal lattice of precipitates corresponded to a cubic crystallographic structure of aluminium-rich oxide. The precipitate lattice alignment with the matrix was revealed for at least a part of precipitates. The early stage of nucleation outside thermal treatment is discussed in terms of point defect enhanced diffusion ensuring sufficient atomic transport to allow solute atom precipitation.