Having done a little research on this, I know that option+left arrow and option+right arrow do not by default move by word in the OS X Terminal application. But mine did, until about a week ago.
The only thing that's changed is oh-my-zsh updated. I see that it has
bindkey "^[[1;5C" forward-word
bindkey "^[[1;5D" backward-word
defined but I don't know what those keys are. GitHub's last recorded change to the file was 24 days go and not related to those keys, so this must predate the latest update.
I have also tried the "Use option as meta key" option under Preferences > Settings > Keyboard. That only results in printing [D and [C when I hit option+left arrow and option+right arrow.
One way to fix your issue is to:
Terminal.app > preferences > Settings > Keyboard there's a Key -> Action list.option cursor left and option cursor right and set their values to \033b and \033f respectively.