mother, grandma, old woman
媼 centers on an older maternal figure: a mother, grandmother, or elderly woman. The meanings are closely related, all pointing to a respected older female family member.
媼 combines 女 (woman) with a phonetic component historically associated with warmth or enclosure. The exact origin of the right side is uncertain, but the character likely developed as a phono-semantic compound for an older woman.
A woman 女 beside a shape that looks like a warm sun 日 over a dish 皿: picture a grandmother serving warm food, the classic image of an old woman caring for her family.
For オウ, imagine the old woman saying 'Oh!' (オウ) in surprise when she sees her grandchildren. The 'Oh!' sound cues オウ while the grandmother figure ties it to the meaning.