I looked over my shoulder and there was Nora as white as the dress she had on.

had 是动词 have 的过去式，表示"拥有、穿着"等含义。 on 在这里是副词，表示"穿着、戴着"的状态。 had on 是一个短语动词，意思是"穿着（衣服）"，强调穿着的状态。