From searching the forums it appears that none are sent, but I'll repeat the question here.
1. User A sets user B in the OOF feature
2. A new task is created for User A using a Destination User. In this case the Destination User is grabbed from a SharePoint list.
3. User A receives the notification and worklist item as normal.
4. User B receives nothing, but if they check their K2 Worklist the item is there.
We are using Blackpearl 4.6 (4.12060.1.0) so perhaps this feature has been added since our version, or needs to be activated.