If I understand correctly, you want this task to run as long as possible, filling up all the time between the end of task 13 and the end of task 5? OmniPlan doesn't actually allow for this (yet?). It only deals with tasks whose duration is known (or calculatable from effort and resource assignments).
So task 14 is behaving exactly as I'd expect. Its end date is after task 5's end date, and its start date is after task 13's end date. 1 day is the default duration for any task whose duration hasn't been set to something else.
If this is what you meant, please send a feature request email to
omniplan@omnigroup.com, requesting this. And what would you call these sorts of tasks.
(On the other hand, if I've completely misinterpreted, please try explaining again, either here or in an email to us.)