We just posted OmniPlan 1.5.1 final. I encourage you to upgrade to that version.
http://www.omnigroup.com/applications/omniplan/
The scroll bar on the right edge of the window should scroll the task outline and gantt view simultaneously. If that's not working, or you're seeing something else, please contact
omniplan@omnigroup.com
We did not intentionally remove a scrollbar, so if one's gone missing we'd like to hear about it!