Oh My Papa

Oh My Papa is a side quest in Fallout: New Vegas. Its outcome will affect what the Great Khans do by the end of the game.

Detailed walkthrough
Note: This is a multiple option quest. There are several different solutions.
 * Travel to the Great Khans' base in Red Rock Canyon.
 * Enter the Great Khan longhouse, which is up a cliff on the right, and is the only non-tent building (It will be marked on the Pip-Boy map). During the daytime Regis, Papa Khan and Karl can be found sitting and eating behind a long table.
 * Talk to Papa Khan. Exhaust all chat options. One will learn that the Khans have an alliance with Caesar's Legion. The Courier's orders are to destroy the Khans if they are allied with the Legion. One can do this, but there are also several alternatives.
 * Exhaust all chat options with Regis and Karl. This may require reading Karl's journal found in the unlocked back room within a footlocker.
 * Upon going through all of the various dialogue options, talk to Papa Khan again. A new option will appear, where the player character can formally request that the Khans break off their relationship with the Legion. Papa will refuse.
 * Exit the longhouse. There will be a forced conversation with Regis. He heard one asking Papa to break off with the Legion, and agrees that an alliance with the Legion is not the best course of action for the Khans. He will tell the player character that they can convince Papa Khan to end their alliance with the Legion by obtaining the support of the 4 advisors (Regis, Jack, Diane and Melissa) and discrediting Karl. This officially kicks off the quest.
 * Talk to Regis again, and he will tell the player character that if he were the group’s leader, he would ally them with the NCR instead.
 * (Optional) (NCR): Return to Colonel Moore. If one tells her that the current leader will not support the NCR she will order the player character to assassinate Papa Khan. Accepting this order is the quickest way to end the quest and the only way to have the Great Khans become an ally of the NCR, but one can choose a more diplomatic approach.
 * (Conclusion Option 1) (NCR) Assassinate Papa Khan: Note: If one is doing the main questline with Mr. House or the Independent/Yes Man factions, one must not pick this option (otherwise the Khans will still fight on Caesar's side in the final battle). The player character must complete the quest normally, through convincing the four Khans and performing Speech checks or bringing Papa Khan the book. Once this is done, he can be asked to change his mind and ally with the NCR. He will say no, but one can tell him to stick around a while longer. The quest marker will say to tell Colonel Moore that the Khans are dealt with, but one can instead propose an alliance between the Khans and NCR to her. Following this dialogue will further the quest, with the objective to assassinate Papa Khan. If the player character can kill Papa Khan undetected by the other Khans, Regis will become the leader of the Khans. Regis will then agree to ally the Khans with the NCR. This is the only outcome that aligns the NCR with the Great Khans.
 * Papa Khan can be killed in his room by pickpocketing his room key, picking the hard lock and waiting for him to enter, or waiting until midnight, when he goes to bed. Follow him in and wait until around 2 am while sneaking with a silenced weapon (or melee), reverse pickpocket an explosive, or use the Mister Sandman perk.
 * Another option is to wait until the afternoon, Papa Khan can be found in the outhouse behind the longhouse. One can sneak up on him there and use an explosive or silenced weapon, and the rest of the camp will not become alarmed.
 * Find Regis either in the longhouse or in his tent and follow the conversation. His tent is located near the big fire with the roasting brahmin. It has some skulls in front and an NCR flag inside.
 * (Conclusion Option 2) The diplomatic solution (NCR and Yes Man): One needs to earn the trust of 3 out of 4 Great Khan advisors. With this option the player character can convince Papa Khan to leave the alliance with Caesar and either leave the Mojave' to "claim their own glory" or die in a suicide mission against the Legion becoming something other than "ghosts and martyrs" depending on which dialogue options you choose.
 * To obtain Jack and Diane's trust, one only needs to convince one of them. They are located due North, through the canyon at the Red Rock drug lab. Either pass a Speech check of 60 with Jack, or talk to Diane to start the quest Aba Daba Honeymoon. This quest involves running several errands for her. It is also possible to fail Aba Daba Honeymoon because of several bugs and quest conflicts with related quests Bounty Killer and I Love Bananas; yet one can still earn enough trust by simply teaching Jack all of the chems recipes.
 * To obtain Melissa's trust, visit her at the Great Khan encampment and tell her that women are not allowed to serve in the Legion. Although she requires no extra convincing, reaching her in the first place requires reaching an area infested with deathclaws.
 * To obtain Regis' trust, one needs to find him proof that the Legion makes slaves of their allies. Stealing Karl's journal from his footlocker and telling Papa Khan about it will disgrace Karl (fulfilling the optional part of the quest) and gain Regis' support at the same time. Proof can also be found in Caesar's tent in his fort. It's a slave ledger that one will find on one of the tables.
 * (Optional): Find a way to disgrace Karl in front of Papa Khan. This can be accomplished by telling Papa Khan about Karl's journal, as above. Alternatively, with high enough Speech (at least 75 to get through all 3 Speech checks), Karl can be provoked into revealing his true feelings about the Khans. Do this while inside the Great Khan longhouse, with Papa Khan also present. Either way, the Great Khans will proceed to kill Karl, and the optional objective will be completed.
 * Note: Stealing Karl's journal from his footlocker and then killing him will disable one from using it as evidence against the Legion for Regis, making the slave ledger in Caesar's Tent the only usable proof for this quest. This is problematic if one has not been anywhere near Caesar's camp yet, as there will be no quest pointer once all other objectives for this quest are complete. One can use a console command to manually display the objective to talk to Papa Khan at this point if one wishes to bypass getting the ledger:
 * Papa Khan will reject the player character's accusation if they bring up the slave ledger with him instead of Karl's journal.
 * Upon speaking to Papa Khan after convincing the other Great Khans, one can pass one of two 75 Speech checks to convince him to break the alliance with Caesar's Legion:
 * Choosing the first Speech check ("claim your own glory") will have the Khans evacuating Red Rock Canyon during the Battle of Hoover Dam (yielding Great Khans fame).
 * The second Speech check ("ghosts and martyrs") will prompt them to perform a suicide attack on the Legion at the dam.
 * If one cannot pass the Speech check, the third dialogue choice regarding past inspiration sends the player character to the Followers. Visit Julie Farkas at the Old Mormon Fort and she will point the Courier toward a Follower named Ezekiel, who is located at 188 Trading Post. Ask him about the Khans and he will give the Courier a history book, Pretty Pretty Horsies: A History of the Mongol Empire. If one delivers this book to Papa Khan with the nicer, first dialogue choice, the Khans will leave the canyon during the battle (and reputation with them will increase); delivering the book with the harsher dialogue choice leads to the Khans' suicide mission.

Behind the scenes
The title of the quest is a reference to a song of the same name performed as an instrumental by trumpeter Eddie Calvert in the UK and as a vocal by Eddie Fisher in the US. A well-known cover was performed on television by Jim Nabors as Gomer Pyle.