Homebrew is the missing package manager for OSX. You can find installation instructions at http://brew.sh.
mos is not part of the Homebrew core, so you need to add a tap to install
it. Open your terminal and enter:
brew tap cesanta/mos
As easy as:
brew install mos
This will install the most recent release.
In order to update software to the latest version use:
brew update && brew upgrade mos