Basically yes. But the frequency used for PWM is usually _way_ higher than what should be realistically visible to the human eye - If it was me, I'd probably use a couple of kilohertz.
Not sure how specifically it's done for OLED though.
Is PWM for varying the brightness of each pixel separately, or for adjusting the brightness of the entire screen? How are these two implemented generally on modern screens? Are PWM and FRC complementary or does one supersede the other? You can tell me about LCDs since you know them better.
At least on my Samsung Galaxy S3, turning the brightness to 100% stops the PWM flicker. If the current to each individual sub-pixel is also PWMed, it's done at a much higher frequency than the global brightness control PWM frequency.