oberst
topmost
The superlative of 'ober'; refers to the highest position in a ranking, hierarchy, or spatial arrangement.
Das Buch liegt auf dem obersten Regalbrett.
The book is on the topmost shelf.

topmost
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topmost
The superlative of 'ober'; refers to the highest position in a ranking, hierarchy, or spatial arrangement.
Das Buch liegt auf dem obersten Regalbrett.
The book is on the topmost shelf.

'Oberst' (topmost) is the superlative form of the adjective 'ober' (upper, which is itself already a kind of comparative) and means 'highest'. It is used to refer to the top of a spatial or hierarchical order.
Der Präsident ist der oberste Befehlshaber der Armee.
The president is the supreme commander of the army.
'Oberst' (topmost) is almost always used directly before a noun and with a definite article, because it refers to a unique, highest position. Predicative use (e.g., 'Das Brett ist oberst') is very unusual.
Die Akten findest du im obersten Fach des Schranks.
You’ll find the files in the top compartment of the cabinet.
The word is used both for spatial height and for rankings. Typical collocations are 'das oberste Stockwerk' (the top floor) in a spatial sense, as well as 'die oberste Priorität' (top priority) or 'das oberste Gebot' (the highest commandment) in a hierarchical sense.
Sicherheit hat für uns oberste Priorität.
Safety is our top priority.
upper
'ober' and 'oberst' both describe a position farther up or a higher rank. 'Ober' means that something is in the upper area or stands higher in a hierarchy. But it does not have to be at the very top. 'Oberst' is the highest position in a spatial order or ranking. Learners may confuse the words because they look similar and both have to do with 'oben' (up / above). The simple difference is: 'ober' means higher or located above, while 'oberst' means at the very top. For example, you say 'die obere Schublade' (the upper drawer), but 'die oberste Schublade' (the topmost drawer) if there is no drawer above it.

Zu Hause stelle ich die alten Fotos in das oberste Regal, weil dort noch Platz ist.
At home, I put the old photos on the topmost shelf because there is still space there.
Moritz legt den Pullover morgen in das obere Fach, nicht in das untere.
Tomorrow, Moritz will put the sweater in the upper compartment, not the lower one.