The most famous one is:
"I am become Death, the destroyer of worlds."
[source] In 1965, he was persuaded to quote again for a television broadcast: We knew the world would not be the same. A few people laughed, a few people cried. Most people were silent. I remembered the line from the Hindu scripture, the Bhagavad Gita; Vishnu is trying to persuade the Prince that he should do his duty and, to impress him, takes on his multi-armed form and says, 'Now I am become Death, the destroyer of worlds.' I suppose we all thought that, one way or another.
which is from,
श्री भगवानुवाच कालोऽस्मि लोकक्षयकृत्प्रवृद्धो लोकान्समाहर्तुमिह प्रवृत्तः। ऋतेऽपि त्वां न भविष्यन्ति सर्वे येऽवस्थिताः प्रत्यनीकेषु योधाः
BG 11.32 - Sri BhagawAna told: I am the expanded death(KAla) for the destruction of the worlds (loka); For hunting people here, I am activated; Except you (PAndava), all the soldiers situated in the confronting sides will not exist.
Additionally, after seeing the splendid effects of the atomic bomb explosion during the test, Oppenheimer recalled the following verse as well:
BG 11.12 - Should the effulgence of a thousand (several) suns blaze forth simultaneously in the sky, that might be similar to the radiance of that exalted One.