[nick101]

Calendar - fixed time event, like a meeting
Omnifocus - task list, or set of tasks (project), overview of my workload
Omniplan - specific formal project plans only, multiple linked tasks and dependencies, multiple resources

My job is project management. I use Omnifocus for the "outside" tasks - reports, meetings, project setup, admin, planning etc and Omniplan for the project plan itself (well, MS Project in reality, because my clients are universally Windows-based, but Omniplan for out of office updates).

I do use the Calendar for time-critical tasks (.e. block out Monday afternoon for planning) which is non-GTD, but I have to organise my time, not just my work. OF on IOS has workable calendar integration