Is Luke 21 part of the Olivet Discourse?

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That's a matter of definition.
The Olivet Discourse is a significant teaching of Jesus found in the Synoptic Gospels: Matthew 24–25, Mark 13, and Luke 21. Scholars call them the Olivet Discourse because Jesus delivered it on the Mount of Olives, overlooking Jerusalem. In this discourse, Jesus responded to His disciples' questions about the destruction of the temple, the signs of the end times, and His second coming.
Actually, Lk 21 happened in the temple:

37 Every day he was teaching in the temple, but at night he went out and lodged on the mount called Olivet. 38 And early in the morning all the people came to him in the temple to hear him.

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Lk 21 is part of the Olivet Discourse because it contains parallel accounts of Mk 13 and Mt 24-25 which took place at the Mount of Olives. The Mount of Olives was only a km east of the Temple Mount, with the Kidron Valley between them, a 20-minute walk.