You already have Oh My Zsh installed. You'll need to remove '/Users/prashant/.oh-my-zsh' if you want to reinstall. code example
Example 1: oh my zsh git
sh -c "$(curl -fsSL https://raw.githubusercontent.com/ohmyzsh/ohmyzsh/master/tools/install.sh)"
Example 2: oh my zsh git
curl -Lo install.sh https://raw.githubusercontent.com/ohmyzsh/ohmyzsh/master/tools/install.sh
sh install.sh