The new background fetch is actually an iOS 7 feature. Apple describes it as "Intelligently Scheduled Updates". (
http://www.apple.com/ios/whats-new/#multitasking) Rather than OmniFocus (or you) trying to micromanage when new syncs should happen, the OS will try to watch what time of day syncs actually bring in new data and when you use the app to make decisions about when to sync in the background in the future.