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.
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.
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 width; length and breadth; lengthwise and crosswise; vertical and horizontal; the four cardinal points
length and breadth; length and width
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