There are two problems with "안녕하세요, 오렌만이다." First, '오랜만이다' is a correct spelling. Second, pragmatically you cannot use two different honorific forms in one sentence to a same person. '~~(안녕)하세요' is honorific form to use for the older people or formal situation. Whereas '~~(오랜만)이다' is not and you use this between friends etc.