OmniFocus isn’t meant to be a calendar replacement, but rather a calendar complement. The calendar does well when it comes to keeping tabs on events that happen at a specific place and time. However, many of our tasks don’t schedule so neatly—that’s where OmniFocus comes into play.
For example, while your calendar may be really good at helping you remember your dentist appointment at 9 on Thursday, OmniFocus is great at helping you remember to file your taxes (which, here in the U.S. can happen anytime between January and mid-April) or to take the trash to the curb Sunday evening.
A calendar is a great place to remind yourself: “Where should I be, when should I be there, and for how long?” Appointments, meetings, and travel are great examples of things that belong on a calendar.
In contrast, OmniFocus is a destination for tasks that tend to have constraints like “I can’t do this until May 15th,” or “This has to be done before 5:00 PM this Friday”, or “I can only do this from my computer at the office.”
To summarize: the calendar tells you where you need to be and when. In the gaps between those blocks of time, OmniFocus tells you what you want to get done, and it can help you figure out what’s the most important thing to do in that block of less-scheduled time.