The best replacement for omniplan is merlin.

The best alternatives to omniplan are rated as following:
★★★★★  merlin
★★★★  ms project
★★★  omnifocus
★★★  merlin 2
★★  omnioutliner
★★  x plan
★★  fasttrack
★  fasttrack schedule
★  itaskx
★  sg project
  projector
  microsoft project
  sg project pro

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Speaking about omniplan, one probably shouldn't forget about omnifocus.

Other search terms related to omniplan and their ratings are:
★★★★★ omnifocus
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Google Trends

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omniplan
merlin
ms project