I can't see the "add item" button in swagger UI

kawad1 The rendering of `oneOf` in query parameters as you expect is not currently supported in SwaggerUI. Feel free to register an issue at https://github.com/swagger-api/swagger-ui/issues

However, based on your use case, you should achieve all that you need by just having the `deviceIds` as an array.

        - name: deviceIds
          in: query
          required: false
          style: form
          explode: true
          schema:
            type: array
            items:
              type: string

If you really want to have two separate schemas for a single string and an array of strings, then split into two parameters.

        - name: deviceId
          in: query
          description: "Single device ID"
          required: false
          schema:
            type: string

        - name: deviceIds
          in: query
          description: "Multiple device IDs"
          required: false
          style: form
          explode: true
          schema:
            type: array
            items:
              type: string

Hope this helps.