Where do you find kairri and get the oathkeeper keyblade

It depends on which point you are at in which game. Kairi (with only one "r" if you will) in Kingdom Hearts 2--I hope that's what you're talking about because I have not finished Kingdom Hearts 1--ventures to Twilight Town but gets kidnapped by Axel, so your hopes in finding her there are none...way later in the game the Twilight Town group that helps you (Hayner, Pence, and Olette) are at the Old Mansion beyond the forest, thinking of an alternate Twilight Town portal in it. You find it and once you get through to the Castle That Never Was--I like to call it the "Nobody Castle"--after a few battles you get to where you fight Xigbar, Organization 13's number 2. Once you fight him there's a video where you see Kairi again and meet up with her and Riku and you travel the rest of the way together, even though almost every battle after that you get separated from Kairi. I don't know what you obtain once you meet with them, but I think it's that Keyblade. Just keep traveling through the castle to Xigbar, even though it is VERY VERY VERY VERY VERY difficult to beat him. Good luck, though! Hope this helps!