Tutorial Step 13: Level Resources
Resource leveling analyzes your project and figures out the most efficient way to arrange it, taking into account things like dependencies and the amount of resources available.
By default, OmniPlan responds dynamically to changes in your project by Automatic Leveling resources. If you would prefer to control when the project is leveled, you can disable automatic leveling in the Project menu.
If you make some manual changes to the schedule, like rescheduling incomplete tasks, setting up dependencies, or changing resource assignments, you should then level the project again to make sure your resource usage is balanced out.
Two tasks are assigned to use the resource Me and the resource Glue at the same time.
Click the Level button 
in the toolbar, then deselect the Don't schedule any task before date checkbox and click OK.
in the toolbar, then deselect the Don't schedule any task before date checkbox and click OK. 
The second task moves to avoid the resource usage conflict.