across, horizontal, lateral
橫 centers on the idea of something lying sideways or spanning across: horizontal, lateral, or across. It describes orientation rather than vertical alignment.
橫 combines 木 (tree/wood) with 黃, which likely contributes the sound. The original idea may relate to a crosswise wooden beam or something placed horizontally.
Picture a wooden beam (木) laid across a yellow (黃) frame: the beam runs horizontally, spanning from side to side.
For オウ, imagine an old wooden beam lying across a path, and someone shouts 'Ow!' as they trip over it: 'Ow!' -> オウ, and the beam is horizontal.