Defining routes using multiple basePath

Not sure if this is the right forum - is there one specifically for OpenAPI 3.0 questions?

 

Server platforms like Node/Express have a concept of routes for grouping sections of API. I was expecting to see somethine similar in OpenAPI where you could have something like this: 

basePath: /portal
paths: {}
basePath: /internal
paths: {}

Is there a best practice for accomplishing this in OpenAPI?