How to set up custom text shortcuts on Mac?

On an iphone, you can customize keyboard shortcuts to auto-complete a word in the way you want. For example, turn "omy" into "on my way." Is this possible to do on a mac? I have Mac OSX 10.8.5.

Try this:
http://www.ettoresoftware.com/products/typeit4me/

It is possible to change one key to type something different (or "wrong", if you wish), but not possible to change a key-combo to type your chosen sentence from the Apple-provided software. You have to use third-party apps designed specifically for this.

Probably can just use the phone for SMS. It's way easier and more convenient than using a computer.