Oh My Papa

 is a quest in Fallout: New Vegas.

Quick walkthrough

 * 1) Go to Red Rock Canyon, enter the only non-tent building in the area called 'Great Khan Longhouse'
 * 2) You will see Regis, Papa Khan and Karl sitting and eating behind a long table.
 * 3) Talk to Papa Khan. Exhaust all chat options until a long sentence asking Papa Khan to abandon the alliance with Caesar's Legion appears. He will refuse regardless.
 * 4) Exhaust all chat options with Regis and Karl.
 * 5) Exit the Longhouse; Regis will speak to you about how to convince Papa Khan to ally with NCR instead of the Legion - obtaining the support of the 4 advisers (Regis, Jack, Diane and Melissa) & discrediting Karl.
 * 6) To obtain Regis' trust, you need to find him proof that the Legions make slaves of their allies. Stealing Karl's Journal from his footlocker and using it to disgrace him in front of Papa Khan will gain you Regis' support at the same time (using Speech to disgrace Karl will leave you unable to use Karl's Journal for Regis' support). Proof can also be found in Caeser's tent in his fort. Its a slave ledger that you will find on one of the tables in Caesar's tent
 * 7) To obtain Jack & Diane's trust, you only need to convince one of them. Either pass a Speech check of 60 with Jack or find Anders for Diane. He is in Cottonwood Cove, crucified at the entrance of town.
 * 8) To obtain Melissa's trust, simply tell her that women aren't allowed to serve in the Legion.

Detailed Walkthrough
Colonel Moore can be found at Hoover Dam. She will ask you to assassinate Papa Khan. You may accept the task, or reject it, and instead continue with a more diplomatic approach.

To convince Regis you need to give him a book called Legion slave ledger found in Caesar's tent at the The Fort. Stealing Karl's journal also works.

You need to only convince either Jack or Diane to complete their objective. They are located in the northwestern area of Red Rock Canyon, at a location marked Red Rock Drug Lab. Jack can be convinced via a Speech check. Diane requires evidence that Caesar is hurtful to drug runners. She can be convinced by finding Anders for her, in the side-quest Aba Daba Honeymoon.

Melissa is found in the western part of the Quarry Junction area, at a location marked Great Khan Encampment. Simply telling her that women are not allowed to serve in the Legion will convince her to speak against Caesar. *** If you have previously done the Don't Make a Beggar of Me Melissa will no longer be there to talk to, so make sure to do Oh My Papa first.***

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 Great Khans. Do this while inside the Great Khan Longhouse, with Papa Khan also present. You may alternatively tell Papa Khan about Karl's journal. Either way, the Great Khans will proceed to kill Karl, and the optional objective will be completed. If you provide Papa Khan with Karl's journal, this will also convince Regis at the same time. Note that if Karl is disgraced through Speech, you will be unable to use his journal as evidence to gain Regis' support.

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.

Alternatively you can return to Colonel Moore after speaking to Regis and tell her that the Great Khans will help her if Regis is in command; she will then ask you to assassinate Papa Khan. Once you have killed him, speak to Regis and he will tell you that he will break the alliance with Caesar's Legion and help the New California Republic. There is a downside to this option: You will instantly fail the Why Can't We Be Friends quest and you will gain some infamy against the Great Khans. If you kill Papa Khan without being detected then talk to Regis, you will actually gain rep for the Great Khans.

Assassinating Papa Khan is probably the easiest solution to this mission, and can be done relatively easily, even with low sneak, assuming the player has a 75 lockpick skill. The rear door of the Longhouse leads directly to Papa Khan's bedroom, where he sleeps alone. Use a stealth boy (unless your sneak skill is high), pick the (hard) lock, and reverse-pickpocket a grenade onto the sleeping Papa. It sure beats slaughtering radscorpions and cazadores to get to Melissa, and you can do this without gaining Great Khan infamy. Also, this can be done even if Melissa's dialogue is bugged and doesn't require a trip to The Fort.

If you do not want to kill Papa Khan after talking to the Colonel,go back to the long house, and talk to Khan about the treaty. He will talk about wanting to leave a legacy. If your speech is at 75 or higher, you can talk him into accepting the treaty. If not, tell him you will help, and he will send you to speak with the doctor at the Old Mormon Fort. She will tell you about a wandering follower named Ezekiel, who is at 188 Trading Post. Talk to him, and he will give you a history book about the Mongols. Take the book back to Papa Khan, tell him how the Khans can be like them. After he takes the book, choose the answer about him being wise and good luck, you"ll get some exp and build a good rep with the Khans. You also have the option to ask if Papa would be willing to work with the NCR now that he isn't going to help the legion, this will lead you down a quest path were you will have to assassinate Papa Khan so that Regis can replace him as the tribe leader and an alliance between the Great Khans and NCR can be made.

Behind the scenes

 * The title of the quest is a reference to Eddie Fisher's song with the same name. A well-known cover was made by Jim Nabors as Gomer Pyle.

Bugs

 * If you use speech to discredit Karl or convince Papa once the four Khans agree to break away from Caesar, you will be unable to complete the mission, and For The Republic II, it will say you've killed all the Great Khans.
 * This may not be a bug, but the people of the Mojave considering the Khans "migrating" to be as good a solution as "exterminated".
 * If you use speech on Karl you may not be able to use his journals as evidence to convince Regis. The best bet is to get Karl's Journal on his footlocker inside the longhouse BEFORE getting the quest (small karma loss, close the door to remain hidden) then, show it to Papa Khan
 * There seems to be a bug when starting this quest and leaving behind the backdoor (at least there was for me, patch 1.2). Then simply type "resetquest 00140c3a" in the console and talk again with Papa Khan and Regis. The Quest-Triggering dialogue-options are highlighted again.


 * The Great Khan encampment can be empty thus preventing you to get Melissa's approval. If That happens you can use the console through tapping ^ once than click on the ground while the console is open to target a location than type "PlaceAtReticle 00e595c" this will spawn Melissa at the point you let her spawn.

The assumed cause for this is she has either been killed by the deathclaws nearby or, alternatively, if you have completed her quests before taking "Oh My Papa", she will despawn.


 * 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 PC, 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. Althought 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 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 cant trigger Regis coming up to you and making a proposal after you leave the Long House. For the 360, however, you can still get Regis to trigger his dialogue, but Papa Khans 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.


 * Papa Khan just wouldn't appear in the longhouse at all. When the player is outside, the mission marker points inside the building, and vice versa. Reloading an earlier save helps.