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 dialog 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

 * Sometimes after talking to Papa and Regis about the NCR, when you go outside and are supposed to talk to Regis, Regis just looks at you and doesn't initiate the conversation that starts the quest.
 * If the player convinces Papa Khan to break the Great Khan's allegiance to the Legion, random non-player character chatter after completing the quest will still talk about how the Great Khans were "destroyed" and "massacred," even though a peaceful solution was found.
 * If Julie Farkas somehow ends up dead, the quest will be impossible to complete via the book, even if you head to Trading Post 188 and kill Ezekiel for the book. The only way around this is to convince Papa via the Speech option.
 * 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.
 * If you trigger the dialogue where Papa refuses to break the alliance with the Legion BEFORE this quest is started, the option will NOT come back, as it seems to be a one time dialogue option, and so the quest gets locked and is unable to be completed because you can't trigger Regis coming up to you and making a proposal after you leave the Long House. Using the console command will bring back all of the dialogue options needed to trigger Regis although some of them may still be "grayed out" cycling through all of them normally will allow the encounter with Regis outside of the longhouse. For the Xbox 360, you can still get Regis to trigger his dialogue, but Papa Khan's question will still never show up again. Amendment: If you go at night and/or when Regis is not sitting at the table and talk to Papa, Regis will not give you the dialogue necessary to start this quest.
 * If you use Speech checks to get Karl to reveal his true feelings about the Khans, resulting in his death, "For the Republic, Part 2" will update to inform you that you've killed the Khans even though none are dead. This will likewise happen if you convince Papa Khan not to side with the Legion while Karl is still in their good graces, as they will again kill him. To avoid this bug, steal Karl's journal (it's in the footlocker in the furthest room from the table) and show it to Papa Khan. Karl will die, but the quest will proceed as normal.
 * For the Republic, Part 2 may not advance after completing 'Oh My Papa' via the "ghosts and martyrs" Speech option.
 * A marker is not placed at Caesar's tent to get evidence for Regis, nor does Karl's journal complete objective. Confirmed. Karl's Journal is not the proper evidence to complete this quest on player, Xbox 360(so far works fine on some 360 consoles) and the ps3 (correct you are looking for the Legion slave ledger, which can be found inside Caesar's tent at the Fort). There is a marker for the journal, however no new dialog appears. No marker is present for Caesar's tent and it appears the quest cannot be finished (this is NOT true. Although the marker is missing, going to Caesar's tent and picking up the Legion slave ledger will update the quest as intended). However, on the 360, Karl's journal is inside the room next door to Papa Khan's, in a footlocker, and can easily be stolen.
 * If you don't get the Speech option to assassinate Papa Khan from Colonel Moore even if you have the Journal entry: "Talk to Colonel Moore about a Great Khans/NCR alliance," just go into Papa Khan room's while he is sleeping around 1:00am and kill him (sniper rifle or 12.7mm pistol with noise suppressor mod is recommended) then find Regis and he will show you an Speech option about the alliance, then go to Colonel Moore an let her know about it, this way you will get the Great Khans support in the final battle. This may end the quest as failed and will make Regis become hostile.
 * After talking to Regis and obtaining the 'Oh my Papa' quest you may be attacked by some Great Khans who are hostile. Killing them fails the quest. This may be because Boone is your companion at the time as he does remark after killing them "Guess he fought at Bitter Springs". (possibly triggered by Oscar Velasco, if the quest Climb Ev'ry Mountain is completed with him leaving to Red Rock Canyon. Confirmed Boone WILL shoot him on sight if spotted in the wasteland.)
 * Talk to Colonel Moore about a Great Khans NCR alliance. When you head to the Hoover Dam Colonel Moore is in the Offices area the marker will tell you to go to her office but she is not there she doesn't spawn even if you sleep for a few hours or leave and do other quests for a little while.
 * Offering to find proof of female enslavement for Diane will override Regis' quest target, so completing it will also meet his requirement. Convincing Jack through Speech afterward will get all three characters' support, making the trip to Cottonwood Cove unnecessary.
 * You may get the dialog option to show Regis Karl's journal as evidence without actually finding it first.
 * In some cases, the location of Hoover dam may disappear as a location on the map after completing the quest, in For the Republic, part 2.

O mein Papa Mi Padre del Alma Папаша