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It's generally safe to delete all the old versions. Since these are Sneaky Peeks, there's a chance that one particular version might be totally buggy and you'll want to switch back to the previous version. So it might be worth keeping one old version around.

The current version is the one named just "OmniFocus". All the others should be followed by a version number (like The one with the biggest number is the most recent one that you might want to keep around.