My understanding is that "oder" means "or," and "sonst" means "otherwise," or "or else."
The context is another verse of the poem I presented a couple days ago. Both the English and German text are mine.
Wo ist sie, die eine? Auf die wart' ich, diese jahrlang. Eine zum anbeten, nach sehen, SONST/ODER wenn ich ihre Stimme hoere.
Where is she, the someone? Whom I've been waiting for these long years. Whom I'll adore, when I see her. Or hear her voice (ring in my ears).
(The English words in parentheses are only to "take up" the extra syllables; I don't want them translated into German.)
The most common usage of the conjuction "oder" is in the sense of the direct English translation "or".
Sometimes "oder" can also be used synonymous to "sonst" in the meaning of "otherwise":
Du musst um 14 Uhr abfahren, oder du kommst zu spät.
This meaning would not be appropriate in the lyrics given, where "oder" is the best possible translation, but unfortunately adding a syllable.Tweet