Hello!

If I measure a voltage of a AC voltage source using a 1 MΩ Oscilloscope and let's say the measurement is 200 mV(p-p).
Then what will be the measurement if I measure the voltage of the same AC voltage source using a 50Ω Oscilloscope? Is it same?

 

Depends upon the impedance of the voltage source. If the source is ideal (zero ohms impedance) then the reading will be the same. If not, then the voltage will be reduced by the voltage divider caused by the source impedance and the oscilloscope impedance.

 

Crutschow said it. It dependa on the circuit you are measuring which you have provided NO information about. Circuit loading you know. Every test measurement causes some loading of the circuit being measured and affects the measured value. Hopefully it is minimal. If you are measuring something it is your job to understand both the circuit being measured and the characteristisc of the measuring equipment.