P.O. Box in Osaka

Does anyone know a P.O. Box service in Osaka that I can use?

Use Google translate.
http://rental-box.jp/navi/osaka.htm

Public post office one is free, but necessary to clear stringent conditions
and the vacancy rarely goes out in urban areas like big city Osaka.
https://www.post.japanpost.jp/question/679.html

First one is "Private" company's, called "私設私書箱(shisetsu-shishobako)".
Public post office one is just calling "shishobako".
Private" company's one is NOT Post Office, just a substitution service of receving.

If you don't want to or just cannot receive it at home directly, how about using these services?
郵便局留 : Yubinkyoku-dome(tome=止め)
https://www.post.japanpost.jp/service/sisho/index.html
営業所止め : Eigyosho-dome for Yamato, Sagawa mainly.
http://torisedo.com/19926.html
http://www.sagawa-exp.co.jp/service/eiuke/

BTW, for what?
You are a Brazilian celebrity, recive a lot of fan letters every day?

Thank you for the help. I am gonna look into it.

I am studying in Japan, and needed an option to receive packages, since I am spending most of the time at the University.