Quick question about valid chars for Bitcoin address

[BitcoinArsenal]

I tried to create a custom Bitcoin address with a vanity generator. The address should contain a 0 (zero) or capital O. According to the generator both characters are not valid. I instead chose a lower o which worked. But how are these both characters not allowed?

[ranochigo]

To the human eye, 0 looks similar to the uppercase O and uppercase i looks similar to l and 1. To avoid users mistaking the letters, uppercase o,  0,  i and lowercase L are never used.

As to why lowercase O can be used, the size makes it obvious that its a lowercase O.

[HCP]

This link will answer all your questions as to why: https://en.bitcoin.it/wiki/Base58Check_encoding