Operating frequency of microcontroller PIC18f4550
Every register in the microcontroller exists whether you explicitly set a value or not. If you don't explicitly change the value of a register, the microcontroller will happily use the default value. The default values for each and every bit are defined in the datasheet.

For Microchip products, the default bit values for each register are defined along the top of the register description. See the red circled-area in the screen capture below. The OSCCON register defaults to a value of \$0100?000_2\$. The ? represents a read-only bit that has no default value upon power up. Looking at just the Internal Oscillator Frequency Select bits, IRCF, the default bits are \$100_2\$. So your default frequency is 1MHz.