how to go to Namba station

To whom it may concern,

good day.

I am scheduled to arrive on this Friday, December 1, and I will be checking out on Saturday.

Please let me know whether I can directly go from Arima onsen to Namba station via a express bus.

Unsure about Namba, there are direct buses to Osaka:

https://japanbusonline.com/en/CourseSearch/12700130011

https://www.hankyubus.co.jp/en/

Yes, there are buses that leave directly from Namba OCAT bus terminal to Arima Onsen. However, it may be difficult to reserve one in advance without Japanese skills. After arriving at OCAT you can probably just buy a ticket for the next available bus, depending on how late you will arrive.

For you to get to Namba area in the early morning, railway sounds better than bus, though you need to change trains.

There seems no bus running from Arima Onsen directly to Namba Station of subway or Nankai.

For Namba OCAT, even the earliest bus is to depart at 10:45, and the next bus is not available until 16:00.
(JR-Namba Station is just near Namba OCAT, while Namba and Osaka-Namba Stations are apart.)

The earliest 8:40 bus for Umeda takes you in about one hour to the bus stop in front of Hankyu Sanban-gai commercial complex.
You can walk to subway Umeda Station then catch a train for Namba on Midosuji Line.

Thank you so much for your kindness.

It's better to take a railway as you guys recommended.