Musk is suing OpenAI and Altman for breach of charitable trust and unjust enrichment, accusing them of "stealing a charity" by straying from OpenAI's founding mission to build safe artificial intelligence to benefit humanity.
The world's richest person said the OpenAI defendants manipulated him into giving $US38 million ($A53 million), only to go behind his back by attaching a for-profit business to its original nonprofit, and accepting tens of billions of dollars from Microsoft and other investors to grow.
OpenAI has said the organisation is stronger as a for-profit entity, including the nonprofit that is now a shareholder of the corporation, and that Musk simply wanted control.
In his closing argument in the federal court, Musk's lawyer Steven Molo said five witnesses, including Musk, former OpenAI board members and OpenAI's former chief scientist testified that Altman was a liar.
He noted that when Altman was asked during cross-examination on Tuesday whether he was completely trustworthy and did not mislead people in business, Altman did not say yes unequivocally.
"Sam Altman's credibility is directly at issue in this case," Molo said.
"If you don't believe him, they cannot win."
Molo said Musk, in contrast, was committed to ensuring his money would not benefit individual people.
"Elon wanted OpenAI to remain essentially a philanthropic endeavour," Molo said.
"He did not want to shut down the nonprofit."
He accused the OpenAI defendants of breaching the charitable trust Musk created by enriching investors and insiders at the nonprofit's expense, and failing to prioritise AI safety, open-source technology, and follow nonprofit governance practices.
Defence lawyers will give their closing arguments later on Thursday.
Musk is seeking about $US150 billion in damages from OpenAI and Microsoft, which would be paid to OpenAI's nonprofit to further its altruistic goals.
He also wants Altman and OpenAI President Greg Brockman removed from their roles.
Microsoft has spent more than $US100 billion on its partnership with OpenAI, a Microsoft executive testified.
OpenAI competes with AI companies such as Anthropic and Musk's smaller xAI, and is preparing for a possible initial public offering that could value the business at $US1 trillion.
Musk's xAI is now part of his space and rocket company SpaceX, which is also preparing a potential blockbuster IPO.
US District Judge Yvonne Gonzalez Rogers is overseeing the case.
It is unclear when the nine-person jury will begin deliberations.
If there is no verdict before Monday, the judge and lawyers will return to court that day to discuss how OpenAI should be restructured and what damages should be paid if Musk wins.
Gonzalez Rogers will determine remedies and will award none if Musk loses.
Musk's lawyer told jurors on Thursday morning that the Tesla CEO is "sorry he could not be here".
Musk is in China with US President Donald Trump and other prominent tech executives.
with AP
