The crystallization mechanism of transition-metal oxides (TMOs) in a solution was examined based on ZnO crystallization using in-situ x-ray absorption fine structure (XAFS) measurements at the Zn K edge and semi-empirical quantum chemistry (SEQC) simulations. The XAFS results quantitatively determine the local structural and chemical properties around a zinc atom at successive stages from Zn(NO3)(2) to ZnO in an aqueous solution. The results also show that a zinc atom in Zn(NO3)(2) ions dissolves in a solution and bonds with approximately three oxygen atoms at room temperature (RT). When hexamethylenetetramine (C6H12N4) is added to the solution at RT, a stable Zn-O complex consisting of six Zn(OH)(2)s is formed, which is a seed of Zn-O crystals. The Zn-O complexes partially and fully form into a wurtzite Zn-O at 60 and 80 degrees C, respectively. Based on the structural properties of Zn-O complexes determined by extended-XAFS (EXAFS), SEQC simulations clarify that Zn-O complexes consecutively develop from a linear structure to a polyhedral complex structure under the assistance of hydroxyls (OH(-)s) in an aqueous solution. In a solution with a sufficient concentration of OH(-)s, Zn-O spontaneously grows through the merging of Zn-O seeds (6Zn(OH)(2)s), reducing the total energy by the reactions of OH(-)s. Zn-O crystallization suggests that the crystal growth of TMO can only be ascribed to Ostwald ripening when it exactly corresponds to the size growth of TMO particles.