Comment by Animats

It's not oscillating at 50MHz. Look at the waveform, with the big spike in the middle. That's a spike at some lower frequency, wider than the screen, followed by ringing. Need to zoom out the time base some more to see the period of the big spikes. It's no higher than 4 MHZ (the screen is 12 units wide) and possibly much lower. (Assuming that M:20ns on the display means 20ns/grid division. The manual is a bit hazy on that part of the UI.)[1]

The power regulator IC mentioned is normally run at 500KHz. There's a reasonable chance that this is the power regulator spike not being damped out. Easy enough to check with a scope handy.

[1] https://fotronic.asset.akeneo.cloud/pdfs/media/owon_hds242s_...