오래 means ‘a long time’ and 오랫동안 means ‘for long time’, which 동안 means ‘for (time)’. 오래 and 오랫동안 are often switchable.
오래 기다렸어요. I waited for a long time. (=오랫동안)
남자 친구하고 오래 사귀었어요. I have been dating with my boyfriend for a long time. (=오랫동안)
여기에서 오래 살고 싶어요. I want to live here for a long time. (=오랫동안)
오래 앉아 있으면 허리가 아파요. My back is sore if I sit for a long time. (=오랫동안)
* 오래 안 걸릴 거예요. It won’t take long. (It is weird if the verb is 걸리다 which means ‘take time’)
오랜만 (or 오래간만) means ‘do something that you have not done for a long time’. For example,
오랜만이에요. It has been a long time (since we saw last time)
오랜만에 피자를 먹었어요. I ate pizza. (I had not had pizza for a long time)
오랜만에 친구를 만났어요. I met my friend. (I had not seen him/her for a long time)
오랜만에 영화를 봤어요. I watched a movie. (I had not seen a movie for a while)
오랜만에 오셨네요. You came! (You have not been here for a long time)
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