SAN FRANCISCO, Aug. 12 -- Brad Lightcap, the chief operating officer of OpenAI and one of the company's longest-serving executives, announced Tuesday he is leaving after approximately eight years to start a new venture, continuing a pattern of senior departures from the world's most closely watched artificial intelligence laboratory.

Lightcap, who joined OpenAI in the company's early days and rose to oversee both business operations and special projects, described the departure as "bittersweet" in an internal message shared with the OpenAI team Tuesday morning. He has not disclosed what his next project will be or named a timeline for his exit.

His departure removes one of the most experienced operational hands from a company now managi...