The OpenAI investment matches SoftBank's eagerness to deploy cash, with a capital-intensive business model, a person close to Son told CNBC.

Even without SoftBank's deep pockets, OpenAI has had no trouble raising billions in cash. Its valuation has climbed to $157 billion in the two years since launching ChatGPT. OpenAI has raised roughly $13 billion from Microsoft, and it closed its latest $6.6 billion round in October, led by Thrive Capital and including participation from chipmaker Nvidia, SoftBank and others.