If you're using Fish shell with OMF theme designed with Powerline fonts in mind, you'll need to install them on your machine. These are the most clear and cut-down instructions that I've found to work with Ubuntu 16.10 and all credit goes to petercossey's Ubuntu Powerline Install gist.
cd ~
wget https://github.com/powerline/powerline/raw/develop/font/PowerlineSymbols.otf
wget https://github.com/powerline/powerline/raw/develop/font/10-powerline-symbols.conf
mkdir ./fonts
mv PowerlineSymbols.otf ~/.fonts/
mkdir -p .config/fontconfig/conf.d #if directory doesn't exists
fc-cache -vf ~/.fonts/
mv 10-powerline-symbols.conf ~/.config/fontconfig/conf.d/
Restart your terminal and your powerline fonts should be working.