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 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 elongated 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 successfully 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 of any postcode has 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 makes no distinction between zeroes and ohs, it is impossible to judge whether the numberplate FO12ABC belongs to a 27-year-old car or a 3-year-old car! (That is, unless you happen to know that in the older system, the three-number sequence never actually began with a zero.)
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.