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Oscillation generally comes in 3 forms.
Normal oscillation is a rhythmic motion that is generally visible and audible. It is usually caused by PIDs being too high.
Jerkiness is random, and is not actually an oscillation. It is usually caused by too much vibration. Replace or straighten bent shafts, balance and track your props, and if that is not enough and the tuning doesn't fly right, try increasing (doubling) GCS -> Configuration -> Stabilization -> Expert -> Gyro Noise Filtering.
Invisible oscillation generally cannot be seen or heard. The way you know you have it is that all throttle stick positions act as if they were closer to the middle. This can also happen with normal oscillation and jerkinessMotors tend to run hotter and batteries don't last as long as they should. Powerful copters (those that require less than 50% power to hover) will take off with much less throttle than expected. Weak copters (those that require more than 50% power to hover) will need more power than expected to take off, or will not take off even at full power. Note that there have been reports of this also causing motors to run hotTry increasing (doubling) GCS -> Configuration -> Stabilization -> Expert -> Gyro Noise Filtering, but realize that this will make AutoTune create very slow smooth PIDs.
Oscillation can cause AutoTune to fail. If AutoTune internal sanity checks fail, it won't write PIDs. It's also possible to get PIDs that don't fly well. If you have a problem with oscillation, it must be corrected before running AutoTune. This is especially true of invisible oscillations.
This version of AutoTune puts the copter into a special version of Attitude flight mode that shakes the copter and measures the motions. As a pilot, you should be able to confidently fly in Attitude mode because it will be a bit more difficult than normal to maintain control. Tuning also needs a larger area than simple hovering. As a point of reference, a good pilot should be able to do a tuning in a single car garage stall.
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