A laser (seen just to the left of the oscillator tower) sends a light beam off a test mass mirror and onto a graduated ruler.  When the oscillator is excited, a streak of laser light can be seen on the ruler.  Measuring the length of the laser streak gives the oscillator amplitude.  A simple geometrical calibration converts the length of the streak to the oscillator amplitude in radians.

Magneto-Mechanical
Harmonic Oscillator