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. You are given 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 your Pip-Boy map). During the daytime you will see Regis, Papa Khan and Karl sitting and eating behind a long table.
 * Talk to Papa Khan. Exhaust all chat options. You 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. You 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.
 * Exit the Longhouse. There will be a forced conversation with Regis. He will tell you that you can convince Papa Khan to ally with NCR instead of the Legion by obtaining the support of the 4 advisors (Regis, Jack, Diane and Melissa) and discrediting Karl.
 * Talk to Regis again, and he will tell you that if he were the group’s leader, he would ally them with the NCR.
 * (Optional) (NCR): Return to Colonel Moore. If you tell her that the current leader will not support the NCR she will order you 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 you can choose a more diplomatic approach.
 * (Conclusion Option 1) (NCR) Assassinate Papa Khan: Note: If you are doing the main questline Independent/Yes Man faction, you don’t want to pick this option (otherwise the Khans will still fight on Caesar's side in the final battle.) The player must complete the quest normally, through convicing the 4 Khans and performing speech checks or bringing Papa Khan the book. Once that is done, you can ask if he'll change his mind and ally with the NCR. He will say no, but you can tell him to stick around a while longer. Your quest marker will say to tell Colonel Moore that the Khans are dealt with, but you 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 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.
 * You can kill him in his room by pickpocketing his room key, picking the hard lock and waiting for him to enter, or waiting until midnight, when Papa Khan goes to bed. Follow him in and wait till 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. You can sneak up on him there and lob an explosive or use a silenced weapon, and the rest of the camp won’t 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): you need to earn the trust of 3 out of 4 Great Khan advisors. With this option you 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, you only need 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 aren't allowed to serve in the Legion. Although she requires no extra convincing, reaching her in the first place requires you to negotiate a region variously infested with deathclaws, and cazadors.
 * To obtain Regis' trust, you need 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 you Regis' support at the same time. Proof can also be found in Caesar's tent in his fort. It's a slave ledger that you 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 (you'll need at least 75 to get through all 3 Speech checks), you can provoke Karl 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 you from using it as evidence against the Legion for Regis, making the slave ledger in Caesar's tent the only usable proof you can use.
 * Papa Khan will reject your accusation if you bring up the slave ledger with him instead of Karl's journal.
 * When you speak to Papa Khan after convincing the other Great Khans, you 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 fleeing 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 you can’t pass the Speech check, the third dialogue choice regarding past inspiration sends you 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 you deliver this book to Papa Khan with the nicer, first dialogue choice, the Khans will flee the canyon during the battle (and your 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.

Bugs
O mein Papa Mi Padre del Alma Папаша