[tshaddox]

Also, a perfect circle would almost never strike me as a zero.

[TeMPOraL]

The difference between perfect circle, 'o', 'O' and '0' is contextual, since the shape of all four of them is approximately the same. On their own, the characters themselves are often ambiguous in print, and totally ambiguous in handwriting.

[JohnFen]

When I was a kid, we had an old manual typewriter, from the '40s or early '50s. It had no zero key -- you were meant to use an uppercase O for a zero.