This is the 8606th most frequent Russian word.
оппенгеймер
The Russian word "оппенгеймер" refers to "Oppenheimer," typically referencing J. Robert Oppenheimer, the physicist known as the "father of the atomic bomb."
The word 'оппенгеймер' (Oppenheimer) is used here as a proper noun referring to the famous physicist Robert Oppenheimer.
Оппенгеймер был известным физиком.
Oppenheimer was a famous physicist.
In this sentence, 'Оппенгеймера' is in the genitive case, indicating possession or association, used because it modifies 'биографию'.
Я прочитал биографию Оппенгеймера.
I read a biography of Oppenheimer.
Here, 'об Оппенгеймере' means 'about Oppenheimer,' showcasing the prepositional case used with the preposition 'об'.