One of the main causes of high vertical oscillation is over striding. If you are an over strider and (likely) a heel striker, try landing more on your mid foot and having your foot land only a few cm in front of your hips. This is for gradual up and flat running.
All bets are off when running up or down steeper terrain. In those cases I recon the Garmin can’t separate your body’s inter-stride vertical displacement from the intra stride vertical oscillation. The devices are made mainly for folks running on flat ground.