when ai and may first met, ai was wearing oom's uniform so may thought that the twin who saved her was oaboom, not ai-oon. that was her first love and her motivation to keep going, hence she got into a relationship with oom. but then oom found out that the person may actually liked was the one who saved her all those years ago but may was still confused until the first time she saw ai-oon in ep1 when she went to 'break up' with may by pretending to be her sister.