roar
虓 centers on a fierce, powerful roar, like that of a tiger or an angry person. The single given meaning 'roar' captures this intense vocal outburst.
虓 combines 九 (nine) and 虎 (tiger). The exact historical origin is uncertain, but it likely depicts a tiger's roar, with 九 possibly acting as a phonetic or intensifying element.
Imagine nine (九) tigers (虎) roaring together. The combined sound is a deafening roar.
For オウ, picture a tiger letting out a deep 'ow' sound as it roars: ow -> オウ. The nine tigers all go 'ow' at once.