It's not the letter O, but the digit zero (0), i.e. D F Zero colon.
DF stands for Disk, Floppy, zero is the first one and colon always ends a device or volume name in AmigaOS.
So DF0: is the first, i.e. tyhe internal floppy drive in an Amiga. The next one, normally the first external one (unless you have a big box Amiga), will be called DF1: and so on.
Best regards,
Niels