Is there a way to terminate or abandon an OVDC session?
I allowed a colleague to log in using OVDC on my laptop but he left his session locked, rather than logging out. CTRL-ALT-BACKSPACE-BACKSPACE does not appear to work in OVDC. Is there any way to close the terminate session or abandon it?
Even better - is there a way to open more than one OVDC session to the same server at a time using different credentials?
You can run multiple copies of OVDC provided that you use a different "profile" in each one. The profile is where the client identity presented to the server by OVDC lives. You can read about how to create and use multiple profiles in the OVDC docs at http://docs.oracle.com/cd/E27795_01/E27796/html/profiles.html
Most importantly, do not try to find and copy your existing profile file as the basis for a new profile. If you do that you'll probably end up with both profiles presenting the same client identity, and that's not what you need here.
Disclaimer: I work for Oracle, but I do not speak for Oracle
I just tried CTRL-ALT-BACKSPACE-BACKSPACE and it worked for me (OVDC on Mac). Maybe you have to move your "host key" in the settings away from any of those. Mine is set to "right shift". Other than that there is no way that I could think of to do this from the client. You could create a new profile like mentioned above or terminate the active session from the admin gui (or commandline)