We’re pretty sure he is not using an actual oscilloscope to generate the visuals, but rather visualization software that can approximate the output of an O-scope set in X-Y display mode.

[Pekka] set himself up with quite the challenge – use an oscilloscope screen to display Quake in realtime – could it even be done? Old analog scope screens are just monochromatic CRTs but ...

An oscilloscope gives the following display for the electricity from a battery. Figure caption, Oscilloscope trace showing battery voltage: a straight horizontal line at the top of the screen.