I've arranged “O Christmas Tree (O Tannenbaum)" using my 2+2 chord-arranging method for solo piano.
“O Christmas Tree,” originally the German “O Tannenbaum,” celebrates the evergreen tree’s beauty and steadfastness. Popularized in English in the 19th century, it features a simple, memorable melody in a major key and has inspired many arrangements, including a jazzy version by Vince Guaraldi. 
In 2+2 chord arranging, the left hand anchors the harmony, typically playing root and 7th, root and 3rd, or root and 5th intervals on tonic chords. The right hand uses its thumb to complete the chord (with the 3rd or 7th), while the remaining fingers carry the melody above.
2+2 voicings are hand-friendly, balanced, and versatile. The left-hand notes can stand alone as a complete accompaniment, while the right hand adds fills or improvisation. With regular practice, the 2+2 system helps pianists move from guessing at chords to playing confident, expressive harmony in just a few weeks.
I’ve created over 1,400 solo jazz piano arrangements using the 2+2 method and produced 65 tutorials, all available at www.Patreon.com/HarryLikas
I was also the technical editor for Mark Levine’s "The Jazz Theory Book" and a contributor to "The Jazz Piano Book."