[Lizard]

Two approaches:

1) Make the durations of tasks 2 and 4 the minimum time you want your ads running. And assign the resources as you have described. Set Finish->Start dependencies between each step in your chain. OmniPlan won't make tasks 2 and 4 the maximum length, but it will delay tasks 3 and 5 until you're available.

2) Don't make 2 and 4 tasks at all. Instead, set a lead time on the dependencies from 1->3 and 3->5. Those lead times should be whatever minimum time you want your ads running. Assign the remaining 3 tasks to yourself. As in option 1, OmniPlan will delay tasks 3 and 5 until you're available to do them.

As long as tasks 2 and 4 aren't costing you money or using equipment resources that other tasks might want to share, either of these should be a decent approximation. But they don't work if, for example, Craigslist charges you for however long your ads run, and you need to accurately track that in your Plan.