oar, scull, paddle
橈 centers on a long, slender tool used to propel a boat: an oar, scull, or paddle. The core idea is a wooden implement for rowing.
橈 combines 木 (tree/wood) with 堯, which likely contributes the sound. The wood radical suggests the material, making the character represent a wooden oar or paddle.
The left side is 木 (wood), and the right side 堯 looks like a tall, curved shape. Picture a long wooden oar carved from a tree, ready to paddle a boat.
For ドウ, imagine rowing a boat with a wooden oar, and the rhythmic sound of the oar dipping into the water goes 'dough, dough' — like kneading dough, but on the water. Dough -> ドウ.