OAKLAND, Calif. — OpenAI CEO Sam Altman took the witness stand Tuesday in a trial that pits him against Elon Musk over the future of the organization they co-founded more than a decade ago. Altman and Musk created OpenAI together as a nonprofit research center in 2015 and years later had a falling out over control and the creation of a for-profit arm that’s behind ChatGPT. Altman told the jury that he did not understand Musk’s allegation that he and other OpenAI executives were stealing from a charity . “It feels difficult to even wrap my head around that framing,” he testified. He acknowledged that the OpenAI board had used “creative ways to keep it going,” by taking outside investment and creating the for-profit arm, but he said the result has been “one of the largest charities in the world.” Elon Musk arrives at the U.S. District Court in Oakland, Calif., on April 28. Godofredo A. Vásquez / AP file Altman, a 41-year-old St.…