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.)