All or Nothing

All or Nothing is the final main quest in Fallout: New Vegas if the Courier chooses to side with Mr. House.

Detailed walkthrough
The Courier will spawn at Hoover Dam, where immediately NCR Rangers will be fighting the Great Khans (if the Courier did not make them join them in the quest "Oh My Papa") and Caesar's Legion. The tower that must be entered will be the third one on the right. The Courier will see two NCR heavy troopers who will turn hostile if the override is planted, but they can be lied to with a Speech check of 75, allowing the override to be used without having any of the NCR soldiers turning hostile.

Once the Courier has uploaded the override, they should head left until they find the door to the main power plant, where just outside there will be an electrical switch to turn on the power for the Securitron army. The Courier should then head back in, find the key to the exit door and go back to the dam where they'll witness the Securitron army razing all of Fortification Hill to the ground and head east towards the Legate's Camp. Securitrons will be helping the Courier fight their way to the camp.

Once at the Legate's camp, the player character must either kill the Legate and all enemies from Caesar's Legion or convince the Legate to retreat with a Speech of 100 or a Barter of 100.

Once they are done with the Legate's camp, the Courier must head back towards the gate where they must deal with General Lee Oliver and the NCR army. They can either kill Oliver and his men or convince him to surrender the dam with a Speech of 100, Science of 100, or a Barter of 100. After dealing with Oliver, Mr. House will roll up in a Securitron Mk II, tell them Oliver and President Kimball's likely fates, and end the quest and game.