I'm not sure I'm understanding, so if this isn't what you mean, please try again to describe your problem either here, or emailing
omniplan@omnigroup.com .
If you change a task's start date manually, OmniPlan will try to honor that start date even when you make other changes (such as adding dependencies). In the task inspector, the Schedule field will change from "As early as constraints allow" to "On specified date". If you click the "Reset" button there, it will clear that specified start date, and the task will move to wherever the dependencies want to put it.