OmniFocus has the concept of Action Groups, which are sort of like miniature projects within other projects (or action groups). To make one, just indent the grouped tasks under the parent task via the Edit->Outlining->Indent command (command-]) or select a bunch of tasks and use Edit->Outlining->Group to make them into a group. You have your choice of parallel or sequential execution for the group, set by selecting the group's parent and opening the Inspector, or by clicking on the arrows in the outline view.
I made a pair of sample projects with action groups.
The first project has a parallel action group at the front, but is otherwise sequential. I need to run some errands before I can paint the room, but once I have accomplished the errands (which can be done in any order), the remaining steps must be done in sequence.
The second project has a sequential action group at the front, but is otherwise parallel. Building the model airplane must be done in the sequence shown, but I can choose to work on building the airplane or one of the other recreational activities in any order.