When designing the English character set, it was quite a bad idea to make the letter O (“oh”) look so similar to the number 0 (zero).
But what turned a bad situation into an absolute disaster was the introduction of the convention of pronouncing the number 0 as “oh” – as in “double-oh seven”, or the freephone number “oh eight-hundred”. At least if the convention had been to pronounce the number 4 as “dee”, one could potentially resolve the ambiguity by looking at the shape of the character. As things are, a vertically-stretched circle called “oh” can equally refer to the 15th letter of the alphabet or to the number zero.
Sentient beings can often infer whether a given ovoid is intended as a number or a letter from its context. But computers cannot, and in an increasingly computerised world, this is an increasing problem. A phone number entered as O800 123 456 can be succesfully interpreted by a human, but an unthinking machine will reject it as invalid. My friend who thought, for many years, that his postcode was “DE3 OAB” until I pointed out to him that the third character from the end had to be a number, had no trouble receiving mail handled by human operators, but would have struggled to resolve his delivery address on Amazon.
Here’s another tricky area: car registration numbers. Cars built between 1983 and 2001, are identified by a 6- or 7-character sequence comprising a letter, followed by two or three numbers, followed by three further letters. Those built more recently are identified by a 7-character sequence of two letters, two numbers, and three letters. Bearing in mind that the font in which these characters are printed does not clearly distinguish zeroes from ohs, it is quite impossible to judge whether car FO12ABC is 27 years old or just 3 years old. Unless the car is in front of you, that is.
And it’s not just zero and oh that can cause alphanumerical confusion. The uppercase letter I (“aye”) looks suspiciously similar to the number 1 (one), and to the lowercase l (“ell”). In fact, legend has it that the Queen utters sentences like “One would like to watch telly now” simply because she misread the “I” for a “1” a long time ago and has continued the habit ever since.