Well the definition of the OS itself can vary with the different OS out there. The iOS and Android os both seem to be refering to the application layer, UI and kernel of the systems. WebOS is more about the UI and application layer, while symbian itself only consisted of the application layer and the kernel, no UI.
Since symbian is just the kernel and the application layer with the added telephony and basic graphics framework, the talk of the system being outdated and primitive is not at all true. Symbian was originally created with all the foundation needed for manufacturers to add their own proprietary UI to plug into the OS based uikon framework. Sony ericsson went with UIQ for their touchscreen pdas while nokia went with series 60 avkon for their keypad based phones. The avkon ui framework is 11 years old essentially making it the oldest UI and application framework running on smartphones today. Essentially the UI is very old and needs changing but because of how rigid avkon is not much change can take place unless it is ripped out and replaced by a more flexible alternative.
In s3 OS uikon and avkon is both present along with the new qt based Orbit application framework. Orbit is basically a socket for a plug, and that plug will be the DirectUI user interface layer which is what the s4 UI is completely based on.
S3 is the biggest modification of symbian since the binary break back in 2004 which, ironically, was actually a complete change of the symbian kernel... however the ui did not change. This goes to show that the UI itself is different from the OS.
For webos now: it is essentially a set of webpages running atop a linux kernel, nothing fancy.
So there, the definition differ somewhat.