Does Fosc refer to the crystal frequency before or after PLL is applied for PIC18 microcontrollers?
Fosc is the clock eventually with PLL, so with an 8MHz crystal and 4xPLL you would have an Fosc of 32MHz. However, note that this is divided by 4 internally, so that you will be able to handle 8 million instructions per second (MIPS). 

I found this in the 18F2520/2620/4525/4620 datasheet, table 17-3 on page 187. You can see that Fcy, the cycle frequency, equals Fosc/4. Fosc therefore has to be with PLL.