오랜만이야 (casual) · 오랜만이에요 (polite) · 오랜만입니다 (formal)
long time no see; it's been a while
Example
민수 씨, 정말 오랜만이에요! 잘 지냈어요?
Minsu, long time no see! How have you been?
Word parts
오랜만 is short for 오래간만 'after a long while' (오래 'long time').
Good to know
Often spelled wrongly as 오랫만; the correct spelling is 오랜만.