Propwash oscillation is one of the hardest multirotor / quadcopter tuning issues to deal with. Basic steps to dealing with propwash oscillation include lowering P gain, if appropriate, and raising D gain--again, if appropriate. What's "appropriate"? Watch the video and see!
To really kill propwash oscillation, you may need to upgrade your kit. Running at higher gyro sync frequencies (2 kHz vs. 4 kHz vs. 8 kHz) usually helps. Having a more noise-free copter with less filtering helps. Running multishot helps. And having an ESC with a hardware PWM driver such as KISS or BLHeli_S helps.
Just a discussion video, no actual flight examples because I'm slammed with other stuff but I didn't want to wait to get the information out there. Hope it's helpful, and I'll possibly follow up with a flight video later.
