The recent adoption of silicon components by the mechanical watch industry has triggered the search for new time bases to advantageously replace the traditional balance and hairspring oscillator. In particular, flexure-based oscillators manufactured in silicon could be the breakthrough needed to achieve a new level in mechanical watch accuracy. The characterization of these new time bases requires accurate measurements while existing methods suffer from limitations in this context. Indeed, they rely on acoustic signals which make them dependent on the sustaining mechanism and extrapolation to calculate oscillation amplitude. This paper presents a novel technique for the measurement of the instantaneous angular position of a rotational oscillator along with algorithms to extract critical timekeeping characteristics such as quality factor, isochronism defect, and gravity sensitivity. The measuring principle is similar to that of optical encoders: LED light is emitted onto a photodiode through slits etched into the oscillator. This setup has the advantage of using off-the-shelf optical sensors whereas silicon oscillators generally make most optical sensors unusable because of their thinness and reflectivity. This article presents the device and chronometric performance measurements of a watch-scale flexure pivot oscillator silicon prototype. The experimental results are validated by comparison with data from a separate reference sensor.