This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
暀 combines 日 (sun) with 往 (go, past). The exact historical development and original meaning are uncertain.
The sun 日 beside 往 (going) suggests light moving or passing, but the meaning is unclear. Think of it as a mysterious old character for a bright or passing day.
For オウ, imagine the sun going down with a long 'oh' sound: oh -> オウ, as the day passes.