TQiSoft's Webgenetics(sm)

This program translates DNA/RNA sequences into single-letter-coded protein sequences in the format suitable for fasta searching and other analyses.

Note that there are choices for mapping internal stop codons.

* - While an asterisk is a conventional symbol for stop codons, it is also taken as the end of an input sequence by some programs, for example, fasta (the sequence after an asterisk is discarded in such case). So if you want to directly input the translated sequences into other programs, asterisks may not be desirable.
X - The letter 'X' is conventionally used for ambiguous amino acids, so it may not be a good choice either if you like to distinguish stop codons from unknown codons.
O - Then your final choice is the letter 'O', which, unfortunately, is an 'illegitimate' code for amino acids.