哦 (ò) is the neutral interjection for 'oh!' when you realize or understand something—like a calm 'oh, I see.' It's softer and less emotionally charged than 啊 (ā) or 呀 (yā), which express stronger surprise or discovery; 哎 (āi) is for calling someone or mild disapproval, while 哎呀 (āi yā) conveys stronger emotions like dismay or admiration.