
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.

on top
In addition to something; as a bonus or an extra.
Du bekommst das Buch und obendrauf noch eine CD geschenkt.
You get the book and, on top of that, a CD as a gift.
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
Both words can mention something additional to something that already exists. dazu usually refers to an accompanying addition, while obendrauf adds another extra to an amount or offer that is already there. Use dazu for something that belongs with it, and obendrauf for an extra.

Zum Frühstück gibt es obendrauf noch ein Stück Kuchen.
On top of that, you also get a piece of cake with breakfast.
Zum Frühstück gibt es dazu frisches Brot.
Breakfast comes with fresh bread.