Santa Cruz de Óñez was a fort founded by Martín García Óñez de Loyola in May 1594, near the confluence of the Bio-Bio and Laja Rivers on the right bank of the upper reach of the Rele River in Catiray, ten kilometers south of the Bio-Bio. The site was in an elevated location but with a shortage of water. Gold mines were located across the Rele on a stream called Millapoa.
The fort was elevated to the rank of city in 1595 giving it the name of Santa Cruz de Coya, or Millacoya.