It's correct. It would also work without "do". "do" may be added for emphasis or just to fit the meter.
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It's correct. It would also work without "do". "do" may be added for emphasis or just to fit the meter. This line has a poetic feel, of course. It is not something you would say in everyday life. I would put a comma after "tree".
Your source is wrong.
According to:
https://en.wikipedia.org/wiki/O_Tannenbaum
The second verse is:
O Christmas Tree, O Christmas Tree,
You give us so much pleasure!
How oft at Christmas tide the sight,
O green fir tree, gives us delight!
O Christmas Tree, O Christma