Programmers had been putting slashes through zeros long before The Oregon Trail showed up, and my Mom might even have an old, hand-written IBM coding form lying around to prove it. I imagine in this case it is probably OCR gone awry that confused the zeros and Os.

As one who originally took typing class on manual typewriters, I can tell you that every typewriter I’ve ever used had zeros and ones. If one were to tell me that there existed portable typewriters that did not have those characters, I might believe it.