I think you should check the OpenWeatherMap API documentation: https://openweathermap.org/api
I believe the One Call API was introduced fairly recently. The HA Openweathermap integration does not use this API yet. That is being worked on as you can see here: Convert OpenweatherMap Integration to use "one-call-api"
In the meantime, you can use a REST call to query the One Call API. If you think the forecast in the One Call API better suits your needs, just let me know. I can post my configuration for that if you want it.