Composer does not work with oh-my-zsh

You most likely had an alias setup in your .bash_profile or .bashrc file. You will need to move this over to the .zshrc file.

alias "composer"="php ~/.composer/composer.phar"

Just run:

curl -sS https://getcomposer.org/installer | sudo php -- --install-dir=/usr/local/bin --filename=composer

If you have already moved your composer.phar to /usr/local/bin/composer you can use:

alias composer="/usr/local/bin/composer"