Olza  (Czech: Olše, German: Olsa) is a river in Poland and the Czech Republic, the right tributary of the Oder River. It flows from the Silesian Beskids through southern Cieszyn Silesia in Poland and Frýdek-Místek and Karviná districts of the Czech Republic, often forming the border with Poland. It flows into the Oder River north of Bohumín.