I assume it is not actually a
rhythmic oscillation; that it is flops around randomly, with random timing.
This is normal but can be minimized by tuning (increasing) the PIDs. Hand tuning the PIDs is a long discussion. I suggest you Google about it.
If it is actually a rhythmic oscillation, then at least part of the PID is too high.
Also, for the flopping around issue while descending, there is a feature that increases the PIDs when you decrease the throttle that is made for this.
Configuration -> Stabilization -> Bank# -> Advanced -> EnableTPS
If you are uncertain about whether it is an oscillation or random flopping around, you might post a video here for us to see.