It's pushing my OCD is on overdrive!

They should all be filled or all be open. Personally I think all filled looks better.

Some of FA's icons (not sure which ones you're referring to here) come only in -o

hand-lizard-o for example. Looking at it from the other way there's no get-pocket-o as an example.

They can all be filled someone posted the example on the preview site.

Here is the filled doc, which is currently the only unfilled I believe.

http://fortawesome.github.io/Font-Awesome/icon/file-text/

 

 

I've actually already changed that one