Oud-Aa is a hamlet in the Dutch province of Utrecht (province). It is a part of the municipality of Stichtse Vecht, and lies about 13 km northeast of Woerden. The hamlet was first mentioned in 1138 as "juxta lacum qui vocatur A" (translation: Next to the lake called A), and refers to the Aa River which is a tributary of the Angstel River. Oud (old) has been added to distinguish from Nieuwer-Ter-Aa. The postal authorities have placed it under Breukelen, and there are no place name signs. The castle Ruwiel was located in Oud-Aa, however it was destroyed in 1673 by the French, and now contains a farm. In 1840, it was home to 315 people. There also was a local former municipality called Ruwiel.