I have a multimeter, but I'm affraid it averages the readings as I don't get any spikes or drops.
If I connect the display on an arduino nano/uno (powered from USB) and run a simple analog read, calculate the voltage and print the result on the Oled, I get those spikes in readings when reading any source. It doesn't matter if operates on 3.3v or 5v. (I obviously don't exceed those voltages on the analog pin). But...if I just connect the Oled vcc to an outside source (grounds connected), all spikes/drops vanish. The outisde source in that case is an LM317 set for 3.3v powered by a 9v battery. But to spice it up, If I hook up the whole arduino+oled on that regulator, I see the spikes/drops again (the LM317 doesnt't heat up that much considering the V difference). Seems like if the oled and atmega328 are hooked on the same source, it acts up, or it just induces so much noise into the system, that it messes with readings.