Before opening statements began in the trial between Elon Musk and OpenAI on Tuesday, the world’s richest man was already making his case on X, the social media platform he owns.

Mr. Musk went on the attack, posting more than two dozen times on Monday about OpenAI; its chief executive, Sam Altman; and the trial. Mr. Musk, who was one of the founders of OpenAI but left in 2018, pushed a narrative to his nearly 240 million followers that he had helped create the artificial intelligence lab to save humanity, but that OpenAI had lost its way after becoming a for-profit company.

“Scam Altman and Greg Stockman stole a charity,” Mr. Musk posted at one point, using pointed nicknames for Mr. Altman and Greg Brockman, the president of OpenAI. “Greg got tens of billions of stock for himself and Scam got dozens of OpenAI side deals with a piece of the action for himself.”

(The New York Times has sued OpenAI and its partner, Microsoft, accusing them of copyright infringement of news content related to A.I. systems. OpenAI and Microsoft have denied those claims.)

Mr. Musk has long used the platform formerly known as Twitter to shape public opinion around presidential elections, global politics and other issues. In March, he posted on X about another trial, a case in which he had been accused of misleading investors when he bought Twitter in 2022. Mr. Musk tried casting doubts about the case, but a jury eventually found him partly liable for investors’ losses in the deal.

His posts on X are unlikely to affect the jury in his trial against OpenAI, given that jurors are typically instructed to avoid news and social media. But that has rarely stopped Mr. Musk from drumming up conversations on X that reinforce his point of view.