Splitting and isolating a signal

[mark2]

Thanks for the extra info!

If I'm learning/understanding this correctly, that might shed a lot of light on this.

If the l7805 impedance is 0.2 ohms a 680uF cap should give a cutoff of about 1200 Hz.

So perhaps I can add a resistor and use a much smaller cap and get the same noise reduction. I'll give it a try unless I learn this is totally flawed.

It still doesn't solve my issue, but it helps a bit. This project is probably dead in the water from the noise, but I'm not quite ready to give up.

[antonis]

IF noise comes from mains rectification (100-120 Hz) the LPF have to be placed before 5V regulator..
(Both for "to the point" issue remediation and convenient voltage drop across the filter resistor before 5V regulator output..)

[mark2]

I discovered the power noise, and it's not from the OLED.

I came back to this today and found that it was whisper quiet with an attiny85 and its own supply.  The noise was only introduced when I was using an arduino board powered by my computer via USB. And even then, it was only when my keyboard+mouse usb switcher, or HDMI monitor were plugged in. Each of those two adds considerable noise.