Dropping the "v' turns it into a one-syllable word, which is needed for the line to scan properly.
Its pronunciation is close to "oar."
Some versions of this famous Scottish ballad cut the word "over" down to one syllable in the same way:
"Half-owre, half-owre to Aberdour,
’Tis fifty fathoms deep;
And there lies gude Sir Patrick Spens,
Wi’ the Scots lords at his feet!
~ folk ballad (Sir Patrick Spens)