There are two parts to using the PC/Arduino as an oscilloscope. First, you need some data to show. Second, you need an application to show the data.
The Arduino can be used as the source of the data. You need to develop, or use one of the existing implementations of, the program to show the data.
There are advantages to using a real scope, though. Multiple channels overlaid are difficult to manage with the PC as scope. Getting the data to the Arduino, and from the Arduino to the PC are significant time consuming steps.