Not necessarily! While each version of OmniFocus offers a unique experience optimized for the device on which it runs, each version is a fully-featured standalone application.
OmniFocus can work perfectly fine just on a computer, or on your mobile device all by itself. On any platform you can create a fully usable database, and then enter, edit, and complete items. For your consideration, this comparison chart on our product page covers which features are available on each platform.
You’re welcome to start using OmniFocus on a single device. If you want to pick up another edition down the road, you can set up sync at any time to start synchronizing between your various devices.