No, normally it would be a VOR. But there is no reason why it could not be any arbitrary GPS point, intersection etc.
For example, during training we would pick a point in the training area to practice holding patterns. If you cannot easily set the OBS then I cannot see how it is possible to use the SkyView to fly a hold at an waypoint?
Have a look at a steam driven HSI (eg Bendix-King KI-525A HSI)
Notice the two knobs? One is the heading bug and the other is the OBS.
The OBS is used for setting the desired course to anything (0-359) regardless of what is used as the navigation source.