Pretty much says it in the title. It's difficult to tell O from 0 in the typeface being used.
What operating system/browser are you using? It depends on which font your browser ends up picking...on Fedora 28/Firefox, the monospace font is DejaVu Sans Mono, which does visually differentiate between 0 and O, as you can see in the screenshot. In Firefox if you right click on the text, pick show inspector from the menu, and then switch the tab to fonts, it'll tell you which font your browser picked.
We could just generate codes which do not have easy to mistake characters. o (small letter) / O (large letter) / 0 (number) and 1 (number) / l (small L) / I (large i) are typically the things to avoid.