Symptoms
1. When a user adds an out of Office, do the existing tasks get delegated to others or only new ones?
2. When a user turns off out of Office, do the tasks delegated to others get removed?
Diagnoses
When Out of Office is configured, any current task is duplicated on the worklist for Forwarded users.
Technically it's not delegated, as the user who is out of office can also action the tasks.
Resolution
In answer to your questions:
1. Existing and new.
2. They no longer appear in the Forwarding users worklists
Here are a couple of links that you might find helpful:
http://help.k2.com/onlinehelp/K2blackpearl/UserGuide/current/webframe.html_Out_of_Office_Forward_Users_Scenario.html
http://help.k2.com/en/displaycontent.aspx?id=4541