I have been reading One Punch Man manga for a while now, and noticed that there's apparently a webcomic on the internet too. Some people told me that the webcomic is original and the manga is headcanon, is it true?
Firstly, the Manga isn't headcanon (Note:A lot of fan made manga does exist). It is a detailed redrawing of the Webcomic, with extra chapters occasionally drawn in. The Webcomic is way ahead and is more or less the same, word-for-word if compared to the completed chapters of the manga.
I personally recommend reading the manga first and then continuing with the webcomic. However, the webcomic tends to deliver its humor a lot better because of the ridiculous art style.
The original One-Punch Man webcomic was created by ONE himself, who also created Mob Psycho 100.
For the original webcomics, ONE did the "drawing" himself, and his drawing skills are more or less close to 10-year old kids. Later on Weekly Young Jump came to realise the potential of the web-novel and officially did the redraw for the series.
Original Genos and Saitama, with Bang: