Why does OFP say my CD key is bad?

It's very common for people to type similar looking letters and numbers. Examples of mistyped look-alike characters are: 0/O/D, 1/l, 8/B, 5/S, 6/G, 2/Z, etc. Just look carefully at the printed CD key and you should eventually recognize what you mistyped while installing OFP.

If you've got the OFP Game Of The Year (GOTY) Edition, have a look at this related FAQ item as well.