The ability to chop the task into chunks is one of our feature requests. It's already been discussed on the forum here:
http://forums.omnigroup.com/showthread.php?t=7967
If you use rescheduling to move the task, OmniPlan will not count the completed portion when leveling other tasks. However, you do need to re-level the project. OmniPlan will not re-calculate leveling (and move other tasks into the same time span as the completed portion of the task) until you click the "Level" button again.
If it still doesn't behave as you expect, you may want to send us an email with a screenshot or sample file of the behavior you're seeing.
omniplan@omnigroup.com