You had the options `auto_cd` and `cdable_vars` turned on. With `auto_cd`, if you type a directory as a command name, the `cd` command is implied. With `cdable_vars`, if a directory doesn't exist, or a command doesn't exist with `auto_cd`, then the name is looked up in the directory hash table.
As long as you're using the “new-style” (`compinit`) completion system, which oh-my-zsh turns on, the name will be offered as a completion when relevant.