P.s. I think the I-term in your pid control loop is too high on your quadcopter, that oscillation isn't really affecting your flight control too badly but it heats the motors more than a stable tune would which reduces the service life of the permanent magnets in them.

Could be the P-term but the oscillation looks too slow for that, I guess one way to check for sure (assuming you don't have a blackbox on there) could be to increase your D-term and see what the response is