Certain proximity detctor
If you don't want to use rfid, have a look at infrared (ir) beacons.
The idea is to have ir led(s) on the cat switched by a small micro controller. Each cat can have a unique blinking pattern which is detected by the base station. The idea is basically the same like a tv remote(constantly transmitting).

A drawback would be having batteries on the cat.

If oliver looks very different from the other cats (color but maybe also size) a webcam and image processing might be an idea to try out too. Frameworks like OpenCV provide a lot of the building blocks for this task.