You are perhaps both right:

Oh Tannenbaum

That should convince you that he is right.

Oh Tannenbaum

That should convince him he is wrong.

Solution: There are at least three German Christmas carols titled Oh Tannenbaum.

The most popular is Oh Tannenbaum wie grün sind Deine Blätter and that's the Red flag one.
Less popular but also know is Oh Tannenbaum, oh Tannenbaum, Du trägst ein grünen Zweig the second I have linked too.

Where I have gone to copy the midis has even a thrid Oh Tannenbaum song which was unknown to me.

Wolfgang