In OnTime Subtasks are called SubItems. A SubItem has a bunch of cool features. It's hours roll up to the parent item (Story in Scrum), it can have its own workflows, and can be assigned to someone other than the owner of the story.
To create a Subitem in OnTime right click on the Story and choose Add Subitem.
You can also use the keyboard shortcut NS, to create a new subitem when a story is highlighted.