One of the strangest episodes in the history of the tech industry ended as start-up events often do: with a party in San Francisco’s eclectic Mission District.

Late Tuesday, OpenAI said Sam Altman was returning as its chief executive, five days after the artificial intelligence start-up’s board of directors forced him out. At the company’s San Francisco office, giddy employees snacked on chicken tenders, drank boba tea and champagne, and celebrated Mr. Altman’s return deep into the night.

Mr. Altman’s reinstatement capped a corporate drama that mixed piles of money, a pressure campaign from allies, intense media attention and a steadfast belief among some in the A.I. community that they should proceed with caution with what they are building.

Now OpenAI, which for two days appeared to be on the brink of collapse just a year after introducing the popular ChatGPT chatbot, will replace a heavily criticized board of directors with a more traditional group including former Treasury Secretary Lawrence Summers and a former executive from the software giant Salesforce.