This is the 3650th most frequent German word.
oberst
"Oberst" is a German military rank equivalent to "colonel" in English.
In this sentence, 'Oberst' refers to a specific military rank, commonly known as 'colonel' in English, which is a high-ranking officer.
Der Oberst kommandierte die Truppen.
The colonel commanded the troops.
Here, 'oberst' is used as an adjective to describe the location of the apartment as being the highest or topmost one.
Die oberstgelegene Wohnung hat einen tollen Ausblick.
The topmost apartment has a great view.
In this example, 'oberst' is also an adjective, describing the rule as being the most important or foremost among others.