obendrauf
on top
At the very top of something; on top of something.
Er legte das letzte Buch ganz obendrauf auf den Stapel.
He placed the last book right on top of the stack.

on top
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on top
At the very top of something; on top of something.
Er legte das letzte Buch ganz obendrauf auf den Stapel.
He placed the last book right on top of the stack.

The word is made up of 'oben' (above / at the top) and 'drauf' (a short form of 'darauf', meaning on it). So it literally means 'on top of something' and emphasizes the position.
Die Kirsche kommt als Dekoration ganz obendrauf.
The cherry goes right on top as decoration.
'Obendrauf' can be placed flexibly in a sentence. It often appears at the beginning of a clause to emphasize the additional thing, or directly after the noun it refers to.
Er kaufte ein Hemd und obendrauf gab es eine Krawatte gratis.
He bought a shirt, and on top of that there was a tie for free.
You can recognize the figurative meaning 'zusätzlich' (in addition) from the fact that it does not refer to a physical place, but to information, a price, or an action. It is often combined with 'noch' (still / additionally).
Der Handwerker war schon teuer und obendrauf kam noch die Mehrwertsteuer.
The tradesman was already expensive, and on top of that there was also VAT.
In the meaning 'zusätzlich' (in addition), 'obendrauf' is frequently used in spoken language and informal contexts.
Wir hatten schon viel Arbeit und dann kam obendrauf noch ein neues Projekt.
We already had a lot of work, and then on top of that a new project came along.
to that / additionally
'dazu' and 'obendrauf' can both mean that something additional is added. 'dazu' is neutral and fits many situations, for example with another point, another thing, or a supplement. 'obendrauf' sounds more colloquial and often means an extra or a bonus, sometimes with the feeling of something more than expected. Learners may confuse the words because both express 'in addition'. In neutral or formal sentences, 'dazu' is safer. 'obendrauf' fits well when something is added like a bonus.

Beim Bäcker bekommt sie heute ein kleines Brötchen obendrauf.
At the bakery today, she gets a small bread roll on top.
Zum Salat nehme ich noch ein Stück Brot dazu.
I'll have a piece of bread with the salad.