looking for reasonable 1LDK apartment to rent for 6 months

Hi ChengHong29,

Welcome to Expat.com

I invite you to have a look at the business directory > Real estate in Osaka section. It might help you.

Thank you,

Priscilla
Expat.com team

South Osaka is cheapest.  The further you go south, the cost will go lower --- to a point.
You will pay more, obviously for the inner-core of Osaka.  I would push for a traveling allowance from your employer if you have not been offered that up-front.  It will help you defer some travel costs - perhaps all of it.  A pass then can be used anywhere along your work-home travel line, 7 days a week. As well, when you select a station area, the further you go from the station the cheaper it gets --- unless you get closer to another station, so consider a 10+ min. walk from the closest station and you will get a lower price.
I would suggest you also look across the Yodogawa River at places along the Hanshin and Hankyu train lines.  A place like Sonoda or Mukonoso on the Hankyu can be had for a reasonable rate, but it will be small as most are small in Japan.  Gambatte.