According to Wikipedia[1] it looks like the rebranding effort from Swagger -> OpenAPI was in November 2015, but the OpenAPI Specification was only officially renamed on 1st January 2016. OpenAI was founded on 10th December 2015. So it depends how pedantic we are being -- technically on branding, OpenAI predates OpenAPI by 21 days. In terms of the spec itself (as in, the thing they adopted), Swagger v1 was released August 2011.