Where can I read the original webcomic One Punch Man created by ONE, preferably offline in an iOS app or in printed form.
The web-comic of One Punch Man is a self-published work by ONE, and are freely available, in Japanese, on their website
As far as I know there are no printed or digital versions available for purchase. Hence for offline reading I would suggest a app/browser that supports offline reading.
Well, since you're apparently limited to iOS, and PC, I advise downloading all of the webcomic to your computer and reading it at your own pace.