the value will be available inside "self"

you may define your onchange function using the new api as follow

@api.onchange('your_many_to_one_field')

def onchange_your_many_to_one_field(self):

   # if you want to access the many2one it will be inside self

    your_many2one_value=self.your_many_to_one_field

   #you can pass the value to one2many field by using write function

    self.your_one2many_field.write({'column_to_be_affected':your_many2one_value})

Regards,

Husam Mustafa