In an August 2024 lawsuit, Musk accused Altman and OpenAI of persuading him into giving $US38 million ($A53 million), only to see the nonprofit abandon its mission to benefit humanity and instead become a for-profit corporation.
The trial, now in its third week, may determine the future of OpenAI and its leadership, as it prepares for a possible initial public offering that could value the business at $US1 trillion ($A1.4 trillion).
Under questioning from his lawyer in the California federal court on Tuesday, Altman denied Musk's contention that he and OpenAI President Greg Brockman, who is also a defendant, tried to "steal a charity".
"It feels difficult to even wrap my head around that framing," Altman said, and maintained he hoped that "as OpenAI continues to do well, the nonprofit will do even better".
Lawyers for Musk have sought to portray Altman as a liar about his plan for OpenAI.
Musk testified early in the trial: "If you have someone who is not trustworthy in charge of AI, I think that's a very big danger for the whole world".
The trial marks a clash among tech giants, with Musk portraying himself as a defender of ordinary people from the perils of AI and Silicon Valley titans who care more about money.
Musk, the world's richest person, is seeking about $US150 billion ($A207 billion) in damages from OpenAI and Microsoft, a major investor, to be paid to an OpenAI nonprofit. He also wants Altman and Brockman removed from their roles.
OpenAI was co-founded in 2015 by several entrepreneurs, including Musk and Altman.
It has said Musk knew about the for-profit plan before leaving its board in 2018, and is suing because he regrets missing out on potential riches.
OpenAI created a for-profit entity in March 2019.
Asked by his lawyer William Savitt whether Musk opposed the for-profit plan, Altman said "quite the opposite".
Altman recalled Musk once demanding a 90 per cent stake in OpenAI, and said he was "extremely uncomfortable" with ceding majority control even as Musk lessened his demands.
"I had quite a lot of experience with startups, had seen a lot of control fights," he said, citing Musk's SpaceX as an example where founders of well-performing companies consolidated power to ensure permanent control.
Altman also said that while he and other OpenAI leaders wanted to stay on Musk's good side, he baulked at a merger with Tesla, Musk's electric car company.
"I don't think we would have had the ability to ensure that (our) mission was acted on," he said. "Fundamentally, Tesla needs to serve its customers and sell cars."
Musk's lawyer Steven Molo cited testimony from a former OpenAI board member that Altman fostered a "toxic culture of lying", and from seven former OpenAI officials who said Altman wasn't trustworthy.
"Have you misled people when you do business?"
"I do not think so."
Altman also testified about his brief ouster from OpenAI in 2023, when its board challenged his candour and defended its need to benefit humanity.
He said he considered not returning and moving to Microsoft, but OpenAI meant too much to him.
"I was willing to run back into a burning building to save it," he said.
Altman does not own OpenAI equity directly, but has a stake in a fund invested in the company.
The trial comes after OpenAI raised hundreds of billions of dollars from large technology companies and investors to add computing power. Altman said OpenAI raised $US175 billion from private investors across its lifetime.
Musk's departure sparked mixed feelings within OpenAI, Altman said, with some people worried it might impede funding, while others were relieved to be freed of Musk's demand that researchers regularly defend their progress.
OpenAI's chairman, Bret Taylor, testified that OpenAI received a formal takeover offer from a consortium led by Musk's rival company xAI in February 2025, six months after Musk sued.
"This proposal was to acquire this non-profit by a group of for-profit investors, which felt contradictory to the spirit of the lawsuit," he said.
