I know the vocative ō doesn't elide with a following word in Latin, though it can elide with a preceding one. For example, Catullus LXI.39:
dicite "o Hymenaee Hymen…"
For metrical reasons, this has to elide as dicit'o Hymenae'ymen. And a quick search hasn't found any other cases where ō does elide into a following vowel.
Are there any other words which refuse to elide like this? For example, do all interjections refuse to elide into what follows them? Or is ō unique?
(Notably, the Greek equivalent ὦ can join into the word after it, which makes me think this exception isn't borrowed from Greek.)