Last March, months before the meltdown at WeWork, Masayoshi Son worked through the prospects for another one of his favorite portfolio companies — a startup from India called Oyo.

In a spacious conference hall at his Tokyo headquarters, the billionaire huddled with lieutenants from the startup and his own SoftBank Group to brainstorm strategy. He figured Oyo had the potential to disrupt both the staid hotel business and short-term apartment rentals in Japan, according to people in the room.

One bullet point scribbled on a floor-to-ceiling whiteboard, in particular, caught Son's eye: a target of 1 million rooms within a year. In a burst of enthusiasm, he had everyone sign off on the goals right on the whiteboard, scrawling signatures under the words "BINDING" in all caps, according to a copy seen by Bloomberg News and the people present.