  Witnessing quantumness in mesoscopic objects is an important milestone for
both quantum technologies and foundational reasons. Cavity optomechanics offers
the ideal system to achieve this by combing high precision optical measurements
with mechanical oscillators. However, mechanical quantumness can only be
established if the behaviour is incompatible with any classical description of
an oscillator. After explicitly considering classical and hybrid
quantum-classical descriptions of an optomechanical system, we rule out
squeezing of the optical field as such a witness by showing it is also
predicted without quantizing the mechanical oscillator.
