sideways, side, horizontal, width, woof, unreasonable, perverse
横 centers on the horizontal dimension: sideways, width, or the crosswise direction. The weaving term 'woof' fits this lateral thread idea. The 'unreasonable' or 'perverse' sense extends from going against the expected straight or upright path.
horizontal (as opposed to vertical)
Yokohama (city)
crossing (a road, river, etc.); cutting through (e.g. of a river through a town); cutting across; intersecting (e.g. a railway track)
yokozuna (highest rank in sumo); grand champion
embezzlement; misappropriation; usurpation
to lie down; to stretch out
to cross (e.g. road); to traverse
being rampant; being widespread; being prevalent
snatching; stealing; seizure; usurpation
violence; oppression; high-handedness; tyranny; despotism
profile; face in profile; face seen from the side
sidelong glance
length and breadth; length and width
length and width; length and breadth; lengthwise and crosswise; vertical and horizontal; the four cardinal points
toppling sideways; turning sideways; falling over sideways
sidewise crawl (e.g. of a crab); crawling sideways
arrogant; haughty; insolent
side; beside
horizontal banner
writing horizontally; horizontal writing
byway; side street; cross street
horizontal script (esp. of a European language); European writing; Roman letters
horizontal line
laziness; shirking
横 combines 木 (tree/wood) with 黄, which likely contributes the sound and the idea of something crosswise or horizontal, perhaps originally referring to a crossbeam or horizontal wooden piece.
A wooden beam (木) laid across a yellow field (黄) creates a horizontal line. That sideways placement gives 横 its core meaning of horizontal or sideways, and when something goes against the upright norm, it becomes unreasonable or perverse.
For オウ, imagine a horizontal wooden beam that makes a deep, resonant 'oh' sound when struck: 'oh' -> オウ, echoing across the sideways plank.