Most Helpful Girl
Organ is harder. To play properly, you have to occasionally use both feet on the pedals. That is a whole new dimension over a piano or keyboard.
The loudness is controlled usually by the left two "Accelerator" pedals. The right one is usually a crescendo pedal effectively adding stops towards full organ. The little white buttons on the key slips are combination pistons that select preset combinations of stops. Same for the toe pistons either side of the three volume pedals. On theatre organs, some of the toe pistons control sound effects like bass drum, cymbal crash, motor horn, bird whistle etc.
On a 5 (or more) manual theatre organ, that is a whole lot more than just playing!