The title basically says it all - in Vim, my tab's show up as ^I, and I'm wondering if anybody knows why this might be. This is especially annoying when using :o to open a file, as the autocomplete does not function. I've looked all over but haven't been able to find anything, it even occurs with a blank .vimrc file. Is there a setting that would cause this, or a way I can remap the key to get it to function properly?
If I read you correctly, you are talking of autocompletion in command mode when e.g. edit a file.
The section "cmdline-completion" of Vim manual says:
You should check this option.
OTOH, if :o means "open" command, it's obsolete and not recommended, and I'm wondering why do you use it. Really it's not to open a file, it's to enter visual mode.
If I understood you in a wrong way, please elaborate your question with more details. Currently it's too ambiguous.